Here are my suggestion about design. I paint in Artstudio on daily basis half a year, so its not theoretical but practical vision.
Here on screenshot my ideal sidebar. At the moment, layer panel and color panels are too big and it is not possible to place in a row reference/navigator panel. All unnecessary buttons can me moved under submenu to make panels smaller.
It should be pin lockers on every panel. You can move them accidentally, you can close them accidentally by pressing color icon instead of brush icon. I’d like to have this sidebar all the time without changing.
I think changes above is necessary for cleaner design and usability.
And menu in one line on top bar isn’t necessary, but would be good to have an option to place it like on screenshot. It should be faster and convenient to operate with commands, then placing these TRA, ELA, CLE, INV, etc in top bar. These abbreviations are hard too remember, after several hours of painting I found that it is easier for me to use these actions from quick menu. But still ability to put transform icon on a top bar is cool thing.
Thanks.