really cool idea, I like the visuals and the concept. I just lose way to often because I suck at blackjack :D
I think to make this really work the concept should get away from the strict "21" rule and just use the "do I pick a card or do I leave it". This would allow for power ups that decreases the chance of picking to much.
Maybe something like the closer you get to 50 the more points you get. And power ups could be you can choose one card that subtracts instead of add to the sum
You could even create Balatro like jokers that move this artificial line.
But this is more for a full game, really great work for a 7 day project :)