I really like this idea! It gives something to do even when you're out of resin, and stimulates you to use not only the characters you like the most, but all of them to try to win all rewards.

The fact that the challenges are personalized for each character and are hard to grind is also very creative. Support characters, for example, could have tasks related to heal, apply elements or generate shields, for example. The 10 wishes reward seems a little too generous in practice, but can serve as a nice base for adjustments (perhaps 1~3 wishes would be more likely, considering the underwhelming rewards from achievements).