These are really important points!
The UI is most certainly going to be leaving. It's what we are working on right now, but it's a doozy of a workload! The big goal is make this into a game that doesn't just bleed "MADE IN RPG MAKER" like most games of a similar style. At the same time, we have such a story to tell with this! The menu is very likely going to be simplified to basics like inventory, map and options.
The sprite does seem a bit out of place. I could never really tell why, but that makes it clear. I'm not sure if I should use a more detailed tileset or if I should change the sprite to be more fitting to the world it's in. The portraits are the main focus, so maybe having a more simplistic sprite would balance it out a lot better.
if you really want to get into pixel art things, you should look at the difference between 8-bit graphics and 16-bit graphics. right now your tileset is 8-bit, quite reminiscent of some of the older pokemon games for instance. some artists prefer a 16-bit aesthetic like the JRPGs on the SNES--that one typically requires a lot of work to get it to look great, since you can't have empty swaths of a color like you have right now for instance.
it's your game so do what you like, but imo you should keep the 8-bit graphics for the overworld and make 8-bit sprites for the characters and items and things to match it. it's much less work and less frustrating for a first time game dev
of course you don't have to do actual 8-bit palettes and whatnot. just use the style. unless again you wanna get really into the nitty gritty of the pixel art. up to you