Grinding game I can play that DON'T have any time constraints on progression

There are some times when I have no time to game at all, and others where I have nothing but time. My exam season is tomorrow and will have a week off and I really want a game I can just get fully absorbed into and keep playing for as long as I can.

An example of this could be pokemon rom hacks, where I can start a new difficult game, play for 30 hours in a few days and beat the game while also grinding and training up my team on the way.

A not great example would be warframe, because I love everything about that game except build times, it ruins the otherwise perfect game for me

I'm not solely against MMO's, but I would rather something that I can play solo or has matchmaking so I don't need to find a group/guild

ty in advance gamers