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.