Terraria. You can choose difficulty of the initial save depending on how familiar the players are, and there are several clear milestones that can serve as transfer point (usually marked by beating specific bosses). In a small world, 20 hours is probably doable? You can maybe find a suitable seed for world generation to be sure.

I was also thinking about Elden Ring - despite the game size, the minimum required amount of bosses to beat the game is surprisingly small, and you can also divide the game into milestones. If it’s too short (like, if your players are speedrunner), you can add the DLC on top.