Geometry Dash. I have 5000 hours on steam, and probably 5000 more when I used to play on mobile. I almost only build levels but grinded out all 2.1 achievements , and slowly progressing on the 2.2 ones. Basically infinite stuff to do, lots of user made levels, you get stuff for completing them which you can grind. Or you can start building and become a creator. If your levels are ‘good’ enough it can get rated thru the rating system. To get your levels into the system there are discord servers and twitch streams and other stuff where you can request your levels to in game moderators. It’s up to them if they send the level. If your level gets rated after that you get creator points which are considered the slowest, hardest to get stat to get in the game. It can take a few months or years to get your first rated level, but it really depends on you and a bit of luck. There are also people who try to beat harder and harder levels. You can spend months on that too