Nice game, maybe make it so that you are automatically fully healed whe you enter the dungeon since asking for rimuru to heal you everytime is just an unescessary additional step.
The first upgrades are a bit steep in price so early game is slow and whenever you encounter the horned slime you just return to the menu and try again because you can't beat that thing without buying the sword and upgrading it a couple time.
Beside all of that we can see the game was made with love and care, pixel art is nice and i don't find it that messy.
Also there was a bug where 2 ennemy spawned at the same time, i could attack them separately and they were both attacking me. I though it was pretty neat and maybe should be thought of as another possible encounter to have multiple ennemies.
Maybe trying to make pallet swaps of ennemies could be a nice and easy way to make a bit more content.
Overall very nice game.