Thanks for giving it a go! Balance is an ongoing struggle for me, I'm not sure I'll ever get it completely nailed down.
That glitch is a hard one to fix. Scrollviews are weird in Unity, but I'll try and get that sorted out.
That's a good idea about the middle mouse button. That isn't being used by anything else at the moment, so it could be used for that. Currently you can move the mouse to the edge of the screen (or use WSAD) but sometimes that isn't convenient.
I tried improving the progress bar to show you just how many bosses are left, hopefully that helps! The gates light up as you complete them.
There's currently no non-random card removal. I've been meaning to add cards for the Thief class which will allow you to discard cards non-randomly, but I've been too lazy to develop that mechanic.
Thanks again for giving it a go!