Yeah, I've learned that 'custom mix' is pretty raw and not exactly a great UX, at least not right now. Which is ok, what I'm doing isn't exactly mainstream.
What I (personally) like about my implementing a 'custom multi' page is that it will be a fairly isolated, seldom-used corner of GCS so my development 'efforts'
won't effect mainstream users while I get up to speed. I.e. I won't be crashing people's vehicles due to FC bugs.
@TheOtherCliff - side question: I'm mulling over evaluating a counter-rotating coaxial prop configuration. From what you know about mixing do you expect any issues with defining two motors at the same XY location but with opposite rotation direction?
Such a custom multi page would also be a good place to experiment with coaxial upper/lower prop throttle bias to find highest efficiency, which is on my to-do list.