Perform: what are your initial thoughts?

[Granyala]

I have tested the instruments. Notes would be easier to read if they had the letters on them.
Colorblindness makes it harder for my partner.

Arrange the bars like a piano. Then you don't need letters.
Real instruments don't have these things either.

which means each time you want to play a new song you have to ditch the old song you worked hard to set up and set up another one.

See above and play in real time.
I agree that they should provide it's own interface, because using existing hotbars to manually plant in 37 actions is a little ... eeh.