Damn, that's a lot of very specific feedback. I appreciate it. I won't respond to all points you've made because most of them are just straight up correct, but I'll address a few:
I've beaten Leprechaun with all build colors (maybe except purple because it's rare and weird). Red is the one that has most blatant combos though, and probably the easiest to play. This might be down to me just knowing the symbols and knowing what to pick though.
The symbol you got after Leprechaun was probably about extra turn tokens - I need to add a tooltip about those
The freedom to walk everywhere is very intended, though I do agree that enemy difficulty might need to be telegraphed better. Maybe I should also add a warning about going to the boss too early if you try.
I'll add mouse controls to combat, it just didn't cross my mind before.
Good points on the negative feedback with the sound and the "nothing" reward, could probably get rid of those