Yeah, from the games I played so far, this one is among my 2 favorites, if not my favorite overall!
I personally think that not all games need to be replayable, it's fine if they make me happy once or twice. Replayability in this case would probably come from more randomness?
But I'm not sure that would be a good fit, because your characters are so thoughtfully handcrafted and the patches fit their personalities so well. I'd hate to lose that part, I think that's kinda the core here. You could of course also handcraft multiple fitting patches + hints per character, but that means a lot of work.
It could also come from continuing the story and some branching, so then I'd want to explore the other story branches on the next playthrough (also a lot of work, and writing, haha). Btw I also liked customers coming back and learning more about them, even from just a short hint (great writing btw, so hard to communicate story/personality with such short phrases).
The difficulty part is a tricky one, because players feel sooooo differently about it depending on their personal skill. Balancing is one of the hardest parts of game design >.<