Thanks for playing! That UI and window size are currently expected, unfortunately, and like you I'm not really happy with how much of the screen the UI takes up. A post-jam build will unlock the window size (it will still render low-res for the effect, but it should upscale nicely), but UI screen estate is a trickier issue. A hypothetical HD PC-only version of the game could get away with a much smaller UI footprint, but I designed the UI with the possibility of touch control on phones in mind, so I wanted to make sure it would work in that worst-case scenario. I think the conclusion is as you say, it's playable, but it could be nicer!
Glad you enjoyed the game otherwise!