Free forming with zynthian is considerably easier in the USB world than it is with cards on the Pi GPIO connector. But power supplies are the real problem. easy to get to a sea of wall warts and extension leads with hum and goodness knows what else.
Your Behringer kit can be driven off a buck converter from the Battery Pack with a separate Buck converter driving the Pi & it’s 5 volt USB world. The 7" touchscreens fit nicely into this world but you have to provide a case of some sort for them and that is an exercise in logistics all of it’s own. But is worth considering as a device to loose into a 19" inch rack That’s kind of how I’m going…
Tablets and VNC let you get a look into the zynth world without having to muck around with 7" touchcreens and make really nice Read only reassurance panels espcially with the new audo mixer interface…
I don’t know if you plan to do the pa from batteries, but you can get to 200watts with small class ds that will run from battery packs (fuse etc)
The modification or manufacture of clips to interface with the battery pack requires a certain deft hand and sadly it doesn’t mount into 19" rack very nicely. It’s the sort of thing people with 3D printer can address but it’s an issue but the battery pack to clip works as a decent low voltage isolator.
Notice the lack of an inline fuse here!! Don’t do this!!
You can get into some big currents so good fuses and wires are to be preferred but sadly most of the low voltage connectors are ghastly horrible barrel connectors and you can only go so far, but as its stage based in need to be rugged.