Pretty neat game! I definitely took the hard way by not reading the instructions provided, but despite my initial bumpy first impression (which I suppose is my fault) I still felt like I got to terms with the game and ended up having a good time, which is quite the achievement!
I feel like the game is very well constructed: love the various animations for actions like having a card get highlighted and float when it is selected, and the effects that act out when you play a card, like how the icons zip up to the top part of the HUD and add to the counters. These elements, along with other subtle things like how the screen shakes and plays a bad sound if you attempt to do something you can't (instead of just giving no response), the way it warns you if you try and end a turn too early, and the contextual control instructions always displaying at the bottom so you know your options, not only make the game feel fun and juicy, actually make this complex game rather intuitive without even needing to consult the outside instructions. Despite my rough start, I felt like I was able to gather all the info I needed to be able to play within the game from trial and error.
It definitely could do a bit more to help, but I understand the Pico-8 template is limiting with the space it has available (I mostly mean screen space, but I imagine data space as well) For example, why does the game even bother starting without just forcing the first draw? I understand not forcing a draw later on since there can still be remaining choices, but I feel like that would help a player see how to start out and hit the ground running instead of starting blanky at these cards they can't do anything with. I felt like you did a pretty dang good job with the limited space already, though.
There were a few bugs I experienced here and there, like where my cursor would remain over an empty card slot and it'd give me options to use it as if it still existed, only for it to not respond to those options and for it to disappear once I move away from it. It was also sometimes tricky to navigate around, for example where sometimes when I go up it takes me to the graveyard instead of the boss which I expected. I also still am not sure I understand the destroy card option.
These card games aren't really my jam so I'm not the intended audience, but I gotta give big props to this game for hooking me in despite all that and the initial confusion: the juicy and reactive menus just gave the game such a good feel to it that I put faith in it and wanted to push on and learn!