Game is really fun. One thing though is that beating a level can cause your progress to slow down. I was able to get 100+ orbs per run in level 2 and then, when I moved to level 3, kept dying after only gathering 40 orbs. The ability to go back to earlier levels could help with this.
You should not incentivise players to farm levels they have already beaten. In fact I wouldn't even allow it. Instead you should make sure the first part of the next level is doable and rewarding for players who did the entire previous level. Rewards don't seem to go up at all with missions, which is odd.