I entirely agree with the points you made! The UI works in a sort of minimalistic, simple way I think, but it can definitely use some sprucing up as it's currently just the default Unity buttons with a color and a nice font. I also would've loved to add some introductory levels to help the player get used to the mechanics and things like creating gaps to move tiles around (as you pointed out), but simply didn't have any time left.
I'm glad you liked it! And you're welcome for the volume controls haha