Fallout 1 and 2

If you go around to all the settlements helping people, you get a different and better outcome for each of those places during the ending sequence of the game. Not sure what that looks like if you go around committing evil, cuz evil playthroughs are whack

I'm growing to really enjoy appreciate the Soulslike approach to items and exploration rewards. I may never use 99% the items, but I want all of them because the descriptions are cool and it's always fun to get something unique. This is all reliant on stuff also being interesting, which the Souls games have managed for me at least.

Hades and Hades 2 have fun extra goals after you start consistently beating runs, but not so many that it takes a staggering amount of time or skill to seal the deal. 32 heat/vows are hard but not wickedly so. Chatting with the gods is consistently interesting as there is a lot of conversations to unlock.

Baldurs gate 2 and it's expansions are a lot of fun if you don't mind the dated game style. Exploring every nook and cranny in the game makes you far more powerful than if you just go through the main quest line. There is a lot of content and interesting stuff to see in that one.

I had a lot of fun completing Celeste!

Far Cry 3 is one of the only games I ever got to the end of and wanted more, so I went to the options and reset all the enemy bases so I could knife hunt them again.

Most games I play though don't really have game conditions I would even regard as "end" or "complete," or if they do I ignore them, like the rocket in Rimworld or Factorio. In Minecraft my self-appointed win condition is typically using whatever modpack I'm playing to make all materials completely infinite and post scarcity rather than exploration. I kinda resent when a modpack forces too much exploration, honestly.

Yoshi's Island is way more fun to 100% than any other way to play it imo

Normal Tunic:
100% Tunic:

Binding of Isaac, if you like roguelikes! Unlocking all the achievements is the true goal of the game, and every time you unlock one, it adds something to the game (even if it’s just something small, like a possible item drop)

Maybe Death Stranding, but only if the game appeals to you. There was something satisfying to me completing all the missions and maxing out all of the delivery locations to get all the little stars on your uniform.

IDK, I did this in 2020 and it really worked for me then. Haven't touched the Director's Cut content, and I don't plan on it tbh

Metal Gear Solid 5 is the opposite and seems like a nightmare to do a completionist run.

Disco Elysium.
Dragon Age Origins (I will find all nugs)
Hollow knight? Though some of the challenges were awful difficult
Assassin's Creed black flag is sort of an inverse of what you're describing. Only the side content is good.
Sid Meier's pirates! Is great fun and there's a lot of side content there too

Hades is a joy to seek out new content in.

I am pretty sure I’ve seen literally 100% of dragon age origins, one of the best rpgs ever.

Animal Well! There's some incredibly obscure and esoteric puzzles in that game. Many of them are completely optional, but they're so intriguing that I wanted to figure everything out anyway. I don't think I'm anywhere near "done" yet, even though I beat the last boss already. You can unlock the ability draw on the map as well, so you can keep track of things.

Also maybe Noita? Completing a run and beating the last boss is basically just the tutorial, that game goes deep. There is an in-game achievement tracker for completing some unbelievably convoluted puzzles and challenges.

While Mario Odyssey is a little tedious to 100%, the other Mario games are awesome for that. The secret levels and puzzles are usually the most interesting and challenging.

The other mainline 3D Mario games have 120 stars is a classic "easy" 100% to get. The Mario 3D Land games and New Super Mario Bros games usually have explicit stats on each level about how many secrets you've missed. Mario 3 and Mario World have more esoteric secrets, but everything is telegraphed if you pay attention to the levels.

I'm not really an achievement hunter, but I did 100% Spiderman and Sunset Overdrive just because they were fun enough for me to want to do everything the games offered. Traveling around the map in both of those games is just so much fun.

Any chance you could set rules for yourself and try to just see credit screens or something? I sorta' get stuck in similar situations but to a much lesser degree. I was trying to beat all the 150 and 200cc ghosts for the MK8DX DLC tracks and gave up halfway through when I realized I wasn't enjoying it and was only doing it because of the completionist urge that's never benefited me.

Running Remnant 2 multiple times to unlock everything is always a good time

Especially unlocking the Archon archetype which took the playerbase data mining the game to figure out exactly how to do it

Elden Ring platinum is easy except for one boss who is utter bullshit. You can save scum to get all endings in one playthrough and if I'm not wrong only there is only weapon you need to pick up that is missable.

Do you like monster catching games? Cassette Beasts and Monster Sanctuary held me for a while. Also, Ni No Kuni to some extent.

Both of the modern Rayman games were pretty fun to collect everything for.

I'd suggest any Monster Hunter game but trying to get everything in some of those will literally take over 1000 hours

It's been a while but I remember enjoying filling out the Assassins Creed 4 map and doing all of the side quests.

I like when I can unlock the fast travel towers for a region and it will tell me where all of the side stuff is. Also love LotR: Shadow of War for similar reasons.

Stardew Valley. You could argue the game's "story end' is completing the community center but there's literally a "Perfection" post game ending

Sidenote complain about the world: Sandbox games combined with my autistic obsessive compulsive need for completionism lead to me playing them with very frusterating habits lmao.

I feel this so hard. I really enjoyed Subnautica, but I think I spent more than half of my playtime farming a giant locker full of every single material because I thought I might need them later, but it turns out that you barely need anything to complete the game and there isn't an endgame to speak of. Whoops. It could have been justified if I were into basebuilding, I guess, but that's not my thing.

Another issue I run into is overoptimization--just figuring out the cheese strat or collecting so much that the game becomes trivial. I think that's why I enjoy Resident Evil so much, since survival horror games are designed for you to be a compulsive loot goblin and therefore don't fall apart even when you collect every last bit of ammo.

Speaking of Resident Evil: with Resident Evil: Village, I didn't get true 100% because I don't like the arcade submode The Mercenaries, but I had tons of fun doing a bunch of NG+ runs to max out all the weapons and get all the unlockable collectibles through the main campaign. I find the process of fighting through the first run with the intense experience of not knowing anything and then continuously getting more experienced and more powerful until you're sprinting through and headshotting enemies with your infinite ammo revolver on the highest difficulty by run six.

Others have mentioned SM64 and Super Mario Galaxy, and I'd also add Super Mario Wonder. The final secret level (unlocked after you complete everything else in the game) is a bit frustrating due to the sparse checkpoints, and there's one or two treasure hunt levels where I had to look up a location or two, but I appreciate that otherwise I could 100% the game just using my platform skill and natural loot goblin instincts. I did my whole run through on Ryujinx and it ran perfectly, too!

One that I wouldn't recommend for this is Control, even though I enjoyed the game and it's got a very cool atmosphere. They have this very unfortunate system where they offer four randomly generated generic missions (Kill X of Y enemy [in Z location] or get X kills with Y weapon/ability) and each mission awards a random selection from a tier and an equippable skill/weapon enhancement. You can see both the exact mission as well as the tier and category of reward before accepting a mission (of which you can have three active) and there's no penalty for dropping a mission. This led me to sitting at the assignment board endlessly regenerating missions until I would get a full loadout of missions with the highest tier rewards and reasonably achievable objectives, after which I'd go do those missions, rinse, and repeat. I would not be surprised if I spent twice as much time doing this than actually playing the game normally, and I sank a good 60+ hours into the game. Oh, the worst part is that the rewards aren't unique and there's unspecified variability within tiers (so a tier VI shotgun enhancement could be like +35% damage, but then you get a +36% damage enhancement and are like, "wait, how high does it go?"). And also, you can only see the broadest category type, so the odds are high that you'll get an enhancement that you don't even want. It's funny--I suspect the devs made the system flexible to avoid forcing people to do missions they don't want to, but it created a perfect trap for people like me.