Simplify and group similar feature, especially the play, mixer, synth features
In v3, the interface starts to be dangerously complicated.
I think it could be simplified by regrouping features. For example:
could be in one "window" that would simplify the menu and related features are regrouped.
The Midi Port / Channel is quite hard to found (same as the "tuning"). But I typically use a simple setup in MuseScore to "get the idea" and print it. Then if I want to work on sound, I use Midi output to a dedicated DAW. Then I don't need mixer, Mute, Solo, Pan, Reverb ... altogether with the MIDI.
It would be nice to have a kind of switch: either I see the Mixer - Audio output OR the MIDI output, but not everything at the same time side-by-side. It would avoid to have the MIDI output hidden behind a [+] button.
My point is that the "main activities" (playback, musical writing, cosmetic details ...) should be clearly visible, instead of having lots of menu and windows.