i was reflecting on this and have another suggestion. i think you could improve the game by going through and giving (literally) everything secondary functions. some examples:
what if the player's hitbox gem were instead a special protective amulet that the main character finds at the start of the game somehow. or maybe the MC is part spirit, and the only way to truly damage them is to shoot their heart. (whats ideal here depends on the rest of the game's universe, but give players a beneficial way to rationalize why they have such a small hitbox, make it something that increases their immersion instead of something awkwardly taken for granted)
the enemies' bullets are another clear example of something whos function seems to be purely mechanical. enemy attacks, beyond simple bullet arrangements, can make the player feel like what they're fighting really is a zombie (make it's attacks feel slow and inanimate), really is a giant dragon skull thing (make it breath fire, but made of scary death energy), really is a slime monster (make it shoot poisonous goo that sticks to the ground, make it absorb some types of the player's attacks, etc)
the gems the player collects, the cards, the crates, the main character's appearance, and im sure so much else, could all be made into something immersive
conversely, teaching the player about lore can have a secondary function of teaching them about new mechanics, creating anticipation for an upcoming boss encounter, etc. for example, if you have a card about broomsticks, and how they're used by some of the game's beings to float, the player isnt simply being told "do x to float", they're learning about and anticipating acquiring a part of the game's world that only in turn adds a new mechanic to gameplay