I liked the upgrade shop in the end of a level. The mana stat feels a little unpolished to me, if I was you I would try to remove mana as bar that reloads and try mana charges. You start the level with 5 points of mana, one spell costs 1 mana, stronger spells cost 2 for example. While traversing levels you find orbs that reload mana points. And in the shop you can buy more base mana or reduce costs of spells. That way you dont have to wait unti your mana refreshes after having missed your spell, or drew for too long!