Just play until you don't feel like it anymore. If you need a milestone, clear a match/run/whatever on the highest difficulty. Or maybe every variety, so in Civ, a victory of each type. Or if you feel like going hard, find a rating you want to get and get there, like Celestial in Strive. I would say just play until you feel satisfied though. Don't worry about what others think of your completion metric. None will satisfy everyone.

Subnautica of course was a banger, but recently I've been insanely addicted to Icarus. The mission oriented gameplay makes the survival gameplay less tedious even though I'm doing similar stuff. You drop down, figure out what you need to complete the mission, set up and get the mission done, then get out. You have meta progression to make missions quicker to work through and you have a levelling system to further impact your strength. Just a super satisfying loop to me.

Arkham Horror 3rd Edition. I was quite apprehensive since I hated 1st edition, but my friend is a huge Lovecraft Mythos fan so it hit the table. When I saw how different it was, I was much less nervous, and I had a good time. We played Focusing wrong for a bit before correcting. We ended up losing, but had a pretty good time. I'd play it again.

Spirit Island: Nature Incarnate. Played with just two people after Arkham. We are more experienced than the other players so we tried the thematic map. Whew, boy can that thing screw you over. Our first game was over in about 3 turns because my spirit couldn't grow to the board all the invaders were in. Attempt two, I replaced one of the growth powers and was able to respond to the early explores this time. My board had a couple explorers at the end of the game as my teammate scrambled to survive. I wish I could help more, but it was enough. I think thematic map is probably better with more spirits.

Probably quite lukewarm at this point, but Gloomhaven. Too much effort to set up and manage, losing often is annoying, losing often with no consequences for losing is even more annoying. It always felt like it would be better as a video game, and guess what? There is one now. It's probably good.

When the big brother dies in Brothers: A Tale of Two Sons. The game is short, but does a great job of getting you emotionally attached to these brothers. Even through the controls, you control both brothers at once with each getting half of your controller. When he dies, it also essentially kills half of your controller. I found myself trying to move the brothers together as I have for the rest of the game.