Challenges are a fun mechanic but they're really deprecated because of their low XP. I know they were originally intended with low XP by design because they reward challenge mats as well...but the value of challenge mats is low enough now that that balance needs to be rethought.
If Challenges had more of their XP moved out of one-time Star bonuses and into their base XP, they would be more attractive to run more than once per life. Or alternatively, what if you could spend a Challenge Token to reset a single Challenge, so you could earn that first-time XP again?