Really interesting game. You definitely accomplished your goal of capturing the roguelite addictiveness into this minigame. I'd definitely consider picking it up if this got a full release.
Some ideas if you decide to expand this would be extra uses for the dice. Perhaps you could use the dropped die as a single-use max HP die. Actually didn't get any HP increases until I beat multiple "bosses".
Luck's usefulness seem to cap out fast so perhaps high enough rolls wrap from rewards from 1d20 to 2d6 kind of like D&D weapons go from 1d12 to 2d6. This would also add some balance since it gives slightly more consistent rolls.
Had a few times I almost got stuck in infinite loops when I focused too much on defense so maybe a give-up option or game speeds up temporarily after 6 rounds.
Unfortunately, the black screen bug can also happen on the buy screen too without changing tabs. Was spending my 100+ points and it blacked out on me. Still worked well enough for the first several runs so got plenty of enjoyment in. :)