Hi, I really like the concept of your game! I liked the mana refill mechanic because it had the effect of tying the card game aspect together with the 2D bullet hell part of the game. Also think the card drawing system is cool and could be something that creates really fun situational battles that make replaying the game super fun.
As for feedback, I guess these suggestions aren't really about what could have been done differently, since the time span of the game jam IS super tight, but maybe rather about what could be added in the future?
What I sort of felt was that which card I chose in the moment didn't really matter all that much. Shield gives me shield, missile launches missile, etc, but really I could spam every card while drawing new ones and just focus on grabbing the mana and they'd all have a loosely positive effect. Maybe the boss might have certain phases where it makes it more advantageous to use one card type over the other? Maybe there are elemental types and respective elemental defenses? Maybe the boss gets an armor HP shield sometimes which could be shredded by a certain attack type? As omalley suggested, a time stop card would be sick for targeting hard bullet hell phases too. While playing a game like this I would expect there to be a lot of moments where I go "damn I wish I had X card type right now but I just used it!"
I guess another thing is that in terms of where my attention is going it's kind of hard to think of dodging the attacks and also choosing which card to use at the same time, and it feels like I'm being mentally pulled away from either one in order to do the other. I suppose an ideal polished version (again, maybe not in the scope of this jam) would either use something like a hotkey system, or a time slow system while choosing, so that there's less "mental CPU overhead" in terms of the flow of dodge -> attack with cards -> collect mana. Perhaps the switch between choosing -> fighting -> choosing is more explicit? As in, maybe when you choose, the boss doesn't attack, and you get infinite time, and after you confirm your cards, there's a battle phase that lasts for a bit where you fully see the effect of your cards? These are just some ramblings that might totally not work with whatever other systems you decide to build on top though lol
Lol sorry, I meant to just take a minute to leave a few short thoughts but it ended up stretching out super super long, it might sound sort of like a rant. But honestly though, I don't usually leave comments or suggestions unless it makes me excited for what a finished version of the game might look like, and I think there's potential in this game that I was drawn to. I've played other games in the jam and a lot of them have polish and "tight controls" and "sick puzzle mechanics" and whatnot but it doesn't seem like it would be something I'd like to play for more than ten minutes at a time. While this game is still sort of bare bones, it's really interesting to me that it played to me like the kind of game that I could daydream about during the day and come up with ability ideas and boss phases and mechanics and stuff for, and honestly I think game ideas that sort of creatively occupy the game like that are actually good ideas.