It's insane how well thought out and polished the game is, honestly, congrats to the entire team! This is a profesional videogame that I'd see myself buying on Steam.
I honestly don't think I have any feedback your team hasn't already considered, but if I could offer my two cents, I'd say maybe rework the way the player gains mana outside of the card mechanics. Maybe discarding cards gives you part of their cost back in mana, so cheap and more support cards could not be exploited for mana refills but the player would have to sacrifice powerful ones instead and wait for their cooldown.
Also, that boss fight is nuts and totally messed up lol I'd love to see someone beat that without becoming immortal!
Fantastic stuff!