I really like the core concept!
Having to manage resources in order to heal provides a great dimension to games like this, where suddenly, your healing does not depend on the world's conditions and whether some enemy dropped a health pickup or not, but rather on the player and his choice to whether spend their valuable resources on healing or not. Some possible pointers on how to further expand/improve this concept:
- Bolts could be used on stores as well, to create a choice for the player whether to spend them on healing or on useful upgrades! Having the items being bought with hearts makes having all hearts filled a optimal course of action!
- Having a mini-game for healing is a valid design option, yet don't forget how this affects the game's flow! Personally, I found it a bit weird to stop in the middle of the action to do a puzzle mini-game without any time limit or a sense or urgency. Also think about the consequences of failing this mini-game if you do choose to include it: Should you heal less or more depending on how well you did? Maybe Spend less per heal in case you succeed? It provides a lot of possible design choices!
Lastly, some UI design tips. If you choose to limit the shooting aim in the cardinal directions, consider allowing players to use the WASD to move and arrow keys to shoot, in a twin-stick like way! I think it would fit the control scheme perhaps a bit better than the mouse, but that's just an opinion.
Solid work on the concept overall! :)