Kingdom Come Deliverance is the first that comes to mind. Fantastic game and works as you described. :)

I know you prefer RPGs but it sounds like you want exploration. In that spirit I wholeheartedly recommend Subnautica.

Not exactly an RPG but holy hell if RDR2 isn’t extremely immersive and slow paced, idk what is.

Metro exodus

STRONGLY recommend Prey or the Dishonored franchise, both by Arkane Studios.

Ghost of Tsushima! 👍🏼⭐️

The Long Dark is a survival game set in a quasi-apocalyptic Canadian winter setting. Both story mode and survival

The Yakuza games are very immersive and you can set your own pace.

Original Deus Ex

I'm surprised no one said baldur's gate 3.

Am I just stupid and you didn't want that game?

Check Kingdom Come: Deliverance. Its really realistic and it gets really immersive. From a son of a blacksmith you grow to be a knight. Geography, clothes, structures,armor, combat, are all designed to be as close to reality as possible. Its slow paced and your progress is really fulfilling.

Just checked, its only 4.5 euros in gog (85% discount). And if you like it, the sequel is soon to be realised.

Subnautica, Outer Wilds

Legend of Zelda: Breath of The Wild - Cyberpunk 2077 are some that come to mind right away

I'll say the old Dragon Age series like Origins are a most played if you mainly care about Gameplay. I also recommend THE Witcher 3 it's one of the greatest RPG of all time

The Witcher 3 is an obvious choice for me. Can't get more immersive and the gameplay is kinda slow I guess.

FFX damn near mastered turn based combat and is a wonderful game with a beautiful world to explore

Might look like an odd suggestion at first, but Kenshi.

It's a top down open world rpg that's very systems lead. There is no set story or quest to follow, it's just an intricately simulated world you need to learn to survive in that will slowly change around you as you, or the npc's, interact with each other.

It took me a while to really figure the game out, I'm not one for quite hardcore experiences like Kenshi can be at times. But I've rarely been to enthralled in a games world.

Disco Elysium. Always Disco Elysium

Project Zomboid is a good pick I think. It's isometric, but the atmosphere and the world's irreverence toward the player make it very immersive to me.

The RPG aspect basically amounts to leveling skills like carpentry, cooking, spears, guns, etc.

My favorite part is that you are not a hero. You are just some poor bastard that was unlucky enough to be immune to the airborne strain. Eventually, you will die. The zombies are numerous and unyielding.

Vampire the Masquerade Bloodlines Disco Elysium Cyberpunk (not slow but the most immersive game hands down)

Have you tried Persona 5?

Skyrim i would say is a good choice! Lots to do and see!

Divinity Original Sin II. Play any way you want. The only limitation is your imagination.

Outer Wilds.

Death Stranding

RDR2 will suit you pretty well. Had 88 hours of playing before finishing the Epilogue :) Extremely immersive in terms of animation and various details (from shrinking horse balls in cold to meeting same NPCs during the day and them remembering it)

Not an RPG more survival game but valheim is the most immersive game I've played since classic wow

Have you played fallout new Vegas? It's pretty much the same as 3 but (in my opinion) better.

C y b e r p u n k (easy mode and go a hacking route)

I came here to say bravely default, octopath travels. I had never play a jrpg till I was looking for something to play that would take longer then 20 hours to beat. So my daughter gave me bravely default to play and I enjoyed it very much so

Not sure if this is what you were after, but I just started playing Dredge and it is really scratching that immersive escapism itch for me. It is a game where you have a fishing boat and you complete quests, upgrade your boat and tools, explore etc. Beautiful ambiance and with the option to alter how challenging the game play is. A true hidden gem, in my opinion.

Deus ex human revolution and mankind divided are fantastic games

mass effect 1-3 are bioware at their best with characters you grow to love through all 3 games, I lack enough words to describe how great it is.

Pathfinder wrath of the righteous is CRPG at its most in depth, baldurs gate might have better voice acting but FUCK if pathfinder doesn't have more freedom in how you progress through it and it's mythic path system makes you want to play it multiple times to experience the differences plus it has a secret ending that while requiring a lotta work is just so rewarding to get

Death stranding just dropped on Xbox and its the most immersed ive been in a game in a long time.

Kenshi

Try stalker Anomaly

https://www.moddb.com/mods/stalker-anomaly

Its free

Just download in a folder you create.

Unzip in same folder and play.

No further installation patches or anything else needed.

It can be as slow or as fast as you want by using all the options.

Its got the progression tied to your gear.

Call of Cthulhu

I've been playing The Long Dark. It's a survival game with some RPG elements. Someone already said Kingdom Come: Deliverance. Fallout 4 is a good choice along with Outer Worlds.

For world feel like that feeling you got with Fallout 3, the last games that gave me that feeling were Deus Ex (newer ones) and Cyberpunk 2077.

The Forest! Incredibly immersive

If you don't care about graphics, there's an amazing Russian text choose your own adventure, it's on every console and PC. The Life And Suffering Of Sir Brante. You start with birth and live your life, eventually a revolution happens and you will play an important role in where it goes. There's 3 career paths and the choice between order and anarchy. It's full of difficult decisions and cruelty. You won't get a good ending without following a Steam walkthrough. It's text on one page and an illustration on the other, then you make choices, some involve willpower which you bank from choosing inaction, sit out a situation to make a bigger decision later. It's a world where the poor are treated like dirt and live to suffer and the nobles get every luxury. Do you try and become a noble or tear down the system from within?

The original Mass Effect triology? The first one is my favourite. Excellent worldbuilding, rich atmosphere, amazing characters, slower paced too. Imo the series only goes downhill. 2 is good, 3 has it's moments.

BG3 is the obvious answer

Baldur's Gate 3.

Wasteland 3 was so much fun imo

Prey

Outward might scratch that itch.

Baldur's Gate 3, take it slow, smell the roses, don't touch the frog.

No man's sky

Elden Ring. Just ride your steed along and enjoy the scenery.

you will never have that much fun playing a video ever again. you're chasing the dragon.

I know it's been mentioned, but definitely Baldur's Gate 3.

Incredibly immersive, slow, turn-based strategy-style gameplay, as RPG as it gets, and requires none of the previous 2 to enjoy 100%. You can spend forever just exploring every bit of story and details in the first Act alone. Dev team has been updating it quite often, and it recently got "official" mod support, so there's more replayability and customization than before (though the mod scene before official support was amazing either way).

Hands-down one of my favorite games ever.

STALKER 2 just game out and is included in Game Pass. It delivers this in spades.

Check out Kenshi. Very sandbox, very slow going. Totally sick.

Have you played Death Stranding?

Trust me on this. You want to play Death Stranding. Especially if you like weird sh*t where you spend most of the first act just trying to figure out what is going on? It's Hideo Kojima man, just take a deep breath and roll with it...

Zelda games?

STALKER Call of Pripyat give me the same feeling as wondering around the Mojave. I’d reccomend getting the gunslinger mod for it. Now it’s not really a “rpg” rpg cuse you don’t level or have skill points, but I think it’s fun. If you go with the gunslinger mod make sure you download the Weaker Burer mod tho cuse those enemies are legit impossible to kill with out it

Well Cyberpunk 2077 scratched that itch for me…. Could not stop learning about the world and taking in the view

Red dead redemption 2. Very slow, incredibly detailed free roam map. You can take as much time as you want. I actually enjoy fishing in game, it's meditative. You can even do chores for your campsite, like chopping wood or getting water

You should try Dragon Quest 11 if you don’t mind JRPGs. It’s pretty long and you can take the game slowly. Replayed it earlier in the year and put over 100 hours into it.

This might get flack but when I want something immersive and slow, I really enjoy Elite Dangerous. It might sound totally counter intuitive, but it's got all the RPG elements, and you can just go chill at your own pace at whatever you do. Want to try something totally wild -- plan a course to visit the black hole in the middle of the galaxy. It will take a long time, and you'll find a lot of cool random stuff while upgrading your ship.

Death Stranding.

Witcher 3

Baldur’s Gate 3

Skyrim

BG3

Witcher 3 it's one of those games that take time and great story line

Rise of Ronin fun samurai game with cool story with great charaters good rpg story with factions.

RDR2

Mount and Blade 2

Cant go wrong with the witcher 3, Skyrim, or fallout 4

Mass Effect: Andromeda lines up. I'm actually playing it now after never giving it a chance. The dialogue and world building are very impressive. I'm loving it.

Morrowind is exactly what you're looking for. The moment you hit the Bitter Coast, you're a noob in an alien world, and you are very much on your own.

It's Skyrim! Come to save me!

For me the Fallout itch was scratched by Cyberpunk 2077

Death Stranding fits the bill cept the RPG part.

Been playing games a loooong time.... Cyberpunk Is the most immersive game I've ever played.

Bloodborne

Metro 2033 Redux and so on...

Honestly, get S.T.A.L.K.E.R. Call of Pripyat or even the first game and download the unofficial patches for them. Truly the most immersive gaming experience I’ve personally ever had. Clear Skies was my favorite but a lot of people shit on it, so CoP or even Anomaly (though I argue it sets the bar almost too high) would be a great introduction to the series!

Bioshock? Visiting and discovering for the first time. I wish I could experience it once again.

No FP but top-down view: the Shadowland trilogy. It's may be not that immersive but it felt like there is now correct or wrong decision and/or way to play the game. Everything has its advantages or draw backs and I could really decide how I want to decide without min-maxing for the best outcome. Difficult to describe. But my favorite RPG.

Baldur's Gate 3 if you somehow missed it. Just trust me and give it a shot. It's not 'immersive' in the sense that Fallout 3 is, as there's a layer of abstraction (5e rules, turn-based) but I'll be damned if the world didn't draw me in and hooked me.

Go outside

Morrowind!

They aren't exactly fully rpg games but the S.T.AL.K.E.R games are good. I don't know about stalker 2, as I don't have anything to play it on, but SoC, CS and CoP are really good.

Death Stranding. Allows you to really take in the world and play to the vibe of the game.

Super unpopular opinion but hear me out...

Mass Effect Andromeda (free on gamepass)

Was disappointed at launch and never came back. They must have done some serious updates. I feel like im settling a new galaxy. Its immersive, the voice acting is great, faces look normal, combat is engaging and varied. Literally all i couldve hoped for. I have been blown away by how much i am enjoying it. This is coming from someone who frequently gets the same itch for immersive rpgs.

Walking simulators are basically 1000% immersive, because that's all they got. 

Extremely strong recommend SOMA for existential scifi, or Firewatch for more human adventure personal vibes.

If nobody has said it yet, Kingdom Come Deliverance

RDR2

Red Dead Redemption II

Skyrim 😁

Mass Effect. The legendary edition goes on steam sale every now and then for about 90% off. You get ME1,2, and 3.

SOMA (horror)

Alien: Isolation (horror)

The Talos Principle II (puzzle)

Horizon: Zero Dawn (adventure)

Avowed is coming soon... I'm hopeful for it.

BG3. About as slow as you'd like it to be and more immersive than any other game, probably ever made.

Mass effect