I see! I don't do well designing puzzles either, so I know what you mean haha
I think that if you add some quests/milestones to the endless runs you can increase the amount of satisfaction the player gets from each run.
So I'm trying to suggest something where every time the player finishes the infinite mode they can at least look at other achievements to feel like they're making progress, instead of the scenario where they're just trying to beat their own highscore