Hey I had a lot of fun with this one. I love deckbuilders so I got the gist of it really quickly. Everything made sense and the mechanics worked well with eachother. Only downside is the same thing that can affect all card games and that's RNG. I came super close to beating the boss but that RNG hit me bad with so many attacks of 25+ damage that I literally couldn't block anywhere close to all of it that they whittled me down just a bit faster than I could stick it out for. Would have loved to try again from the boss but unfortunately it would have meant starting over from the very beginning. Really solid game though, very enjoyable!
I think something that could help balance it is either getting an extra card to choose from after each victory, or the option to remove a card from your deck when you gain a new one (thinning out starter cards). Another option could also be cards that have a currency value when played and a shop you can visit in-between battles to either heal up or purchase more cards for your deck. Maybe even cheap but powerful one-time use cards or something! Anyway, just some ideas!
EDIT: Just wanted to add a link. I played your game while recording and uploaded some of them so the devs can get some better visual feedback. Your game starts at 31:03