Thanks for playing!
I agree that one of the major things I need to work on is balancing, which is why I'm trying to get more and more people playing the game. Since I know the game like the palm of my hand it's very easy to breeze through it but I know that isn't the case for others, so your feedback is really valuable. And yeah I think the boss is too easy/the enemies before it are too hard.
For the final version of the game before you start you'll pick your character, and at that moment it'll very briefly explain that characters mechanic, so hopefully that should help remove some of the confusion around scrap.
As for the controls, I admit I have in mind that players will play with a controller, where you can swap between cards with the shoulder buttons much more easily than on keyboard. I hadn't thought to use the mouse at all, but that is an interesting idea for sure.