Loved the demo, there's really only the visual/audio polish and some quality of life features that still have significant room for improvement, the core gameplay is already really good. I made it to level 17 and accidentally died with the healer that transfers HP, and it was a pleasant surprise to have all these different healers show up after clearing the tutorial.
I found shift-1/2/3 to be a tiny bit unwieldy for controlling the characters, maybe using the row of keys below WASD could be worth considering? Probably the only somewhat lacking features in the game was in commanding units, it'd be nice to be able to control units individually, or at least split them into groups like frontline/backline so I can avoid attacks aimed at my backline without interrupting what the front is doing. Individual-level micromanagement of focus and movement is probably too much to handle for me and most other players, so I'll be happy if I just get a couple of control groups. It could be useful to be able to call up specific abilities?
Visuals are definitely the weakest link of the game, environments are repetitive and dull, and while the character models are nice, they're reused a bit too much. On my first run where I didn't understand how party formation worked I ended up with 3 pointy hat characters, each with slightly different colours. Also, having more enemy variety would be nice, especially if they could better relate to their abilities, because it's hard to pick out who's what when there's a whole group of enemies bunched up together. I don't know if it's easy/feasible to do in Godot, but a more distinct outline for selections might be nice. Attack visual effects are also quite dull, especially the ones where it's just a circle with a noise texture on it. To be fair, a lot of these kind of stop mattering once the gameplay gets hectic enough that I can only look at the health bars, so it's not exactly a major deal breaker.
In general, the UI is functional, but the design and fonts used have some room for improvement. I'm personally not a fan of the fantasy font text, and the backgrounds for a lot of the panels don't look very good.
Metaprogression with the tavern is interesting for giving more choices, but simple bonus stats for everyone seems a bit bland, though I don't really have any better solution. I do think that the initial run through the earlier levels can get boring if there isn't going to be significantly more enemy and environment variety, so maybe some way to skip to a deeper level could be worth exploring.
The 5 minute timer feels a bit short, but it could just be that I'm too inexperienced with the game and its levels since I've only managed to beat the timer once. I also think there's some untapped potential in making it closer to some kind of dungeon crawler and having more non-combat/exploration-related abilities or items, but I'm not sure if the way levels work currently will support that very well.