your awesome man ^^
for the slime, you could just make one long sheet, like with the npc villagers =P
humn, i could give you some tips on your tilesheets too, as they are now theres no real posibility to properly auto-tile with them in Godot. the rpg-maker one you did is a good start! but its lacking some corner and mid tiles =Pi could give you links to two free asset sets with auto-tiling tilesets if you wanna have a look?
https://docs.godotengine.org/en/stable/tutorials/2d/using_tilesets.html#creating...
however has the documents for autotiling, "Creating Terrain Sets (Autotiling)" (quick link in the left bar) has some visuals but eehh, they are boring xD bbut if you do them like that, and someone sets up auto tiling with them, they can do some pretty awesome work in their game :3
but yeah, dont overwork yourself man! burning yourself out isnt good for your health and its detrimental to your work, take your time and have fun =D