Very well done! This is a great starting point, and honestly doesn't seem that far away from being a full game. The only notes I have are that there is some features that are good on mouse that aren't good using keys. For example, the text pop up works on mouse hover, but doesn't work when the same item is focused through key inputs. This led me to use mouse for combat buttons, but keys for switching slimes/enemy focus.
There's also some kind of artifacting when you're in the slime menu. If you hover over an item to see it's description, it populates at the top left corner for the first frame, and then centered over the mouse position.
The only other thing that I think needs some attention is the affinities system. I think it's a good idea, but not fully there yet. Because the weapons you collect do the most damage, the best strategy I found was to make sure everyone had some healing, and lots of weapons. I eventually started dumping all items to one slime, then the next, and so on. While this is fine, it did kind of take the system for granted instead of making it an interesting mechanic. I think, similar to how you start with two slimes and then go on to have four for the rest of the game, starting with one slime, getting loot very specific to their strengths and weaknesses, then adding another, and getting items very specific to their strengths and weaknesses would help show the way the system is intended to be used.
I also think that being limited just to weapons of 3 types is what made that feel disconnected in my brain. Maybe it makes sense as the holy warriors would only have holy weapons to drop, but maybe something along the road could give me an earth weapon, or a water weapon. It seems you may have more plans for the road section since it's pretty empty right now? Not sure, but my assumption is that there's something planned that I can click or interact with to get more items while I travel.
The last thing I'll say is that the boss battle felt great. That was a great challenge, and made me focus a lot more on utilizing the defend, and attack order mechanics. As a tutorial, this seems good. For the next level, I think that having higher health enemies with low damage attacks, or even lots of enemies would make the combat choices feel more consequential.
TLDR, great game, great art, love the direction it's going. Everything else is just thoughts and opinions as someone who enjoys the turn based genre.