I think I do understand what you're trying to do, and I'm saying it's not possible. Or if you do find a way to do it, it'll be practically unusable anyway.
I think you're focusing too much on the installation complexity and trying to minimize that, without realizing yet that the goal itself is irreducibly complex. So by moving complexity away from the installation, you're ADDING it to the operation! You can't get rid of it.
Take a serious look at going the other way: Be more than willing to make the installation complicated, with every part there for a known and understood reason, SO THAT the operation really can be dirt simple.
And don't be married to anything. If you bought something expensive because you believed the ad for it but didn't really understand yet (even if you thought you did), be more than willing to end up with a rig that not only doesn't use it but CAN'T even use it.
xyproblem.info
You seem to be better at the *asking* part of that than a lot of people, providing enough details to see the real problem, but I think your difficulty may lie in *accepting* a solution that completely scraps and reworks everything.
en.wikipedia.org
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As soon as you go beyond the basic, stereotypical, bedroom streamer, OBS's audio becomes a nightmare. So pop it out of there and into a tool that is actually made to do exactly what you want it to do.
Trying to do a professional production job without the professional controls is like chasing windmills. The professional tools are the way they are for very good reasons. Just give up and do it that way. Not necessarily with expensive gear, but with a set of controls that already "just works", even if it's multiple apps.
I have a hard time believing that even a few more *minutes* (that's a lot) of automated startup time would be a problem. You're supposed to have the media gear on, warmed up, and waiting, long before you actually need it. That time is used for last-minute troubleshooting if needed.
If you're first giving it power at the last possible panicked moment, then you've got something seriously wrong with your procedures! And you should expect your operators to wing it with critical functions missing, completely at random and without warning.
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That said, I did have a mobile "guerrilla concert" rig a few years ago, where we really did have about 10 minutes between dumping the bus's air brakes and the first note of a live song from a full band, including the unloading, finding power in a place we've never been before, marking a makeshift stage with speaker stands, etc. That was great fun! But I also did a LOT of prep work beforehand - starting about *two years* before the first show that really worked - to build a rig that was specifically designed to do that.
It also had a custom battery backup for a Pi that I buried in the 6U double-sided rack (12U if you're good at playing tetris inside), so we could just yank the power when we were done, toss it back in the bus, and go to the next stop while the Pi shut down gracefully on battery and then the battery disconnected to avoid further drain. That last part turned out to be a deal-killer for all of the off-the-shelf UPS hats that I looked at. Otherwise I would have bought and used one of those for cheaper.
I still have the electrical schematic, printed circuit board files, and source code for the firmware that went into that custom UPS. All done by me in about a month if you include cheap shipping time.
A big part of what made such a short setup time work, in addition to *lots* of testing the automation beforehand, was to parallelize everything that possibly *could* run in parallel. Set up the "B-chain" first (amps, speakers, everything downstream of the mixer), while the Pi is booting up and running its startup script, so that those two things are ready at about the same time. Then play some canned music from the Pi, controlled remotely from a laptop that had also connected to the rig's embedded WiFi by then (take the plastic case off an old D-Link router so it fits in a rack tray, and cable-extend the antennas to the outside of the rack), both to test the "B-chain" and to attract and/or create some excitement in the audience that was often already there and watching us set up.
Meanwhile, the musicians are setting up their "A-chain" rigs and plugging them into their assigned channels on the mixer and 8-ch headphone amp, so when I'm done with the quick check of each speaker, I take the laptop around the "stage" and tap/strum/activate each individual thing while watching the meters on the remote-control laptop, starting with the faster things to set up while the slower things are still working on it. Everything's still muted while I do that, except the canned music. Then I find a spot in the audience to mix from, which is the band's cue to come back on stage, and I unmute the band and fade down the canned music, which is their cue to start playing live.
Here's the mixer that I used for that:
www.behringer.com
Expand "Software" on the right side, and download the version of X-Air Edit that works on your system. It's completely free and is designed to be the complete user interface for that mixer. And the controls work offline too, so you can play around and see what's there. The mixer is controlled by OSC, as noted above, as is the X32 that it was pared down from, so you can automate either one from Adv. SS if you want.
If you must have physical faders for it:
www.behringer.com
I've done that too, with a crossover Cat-5 from that to a Pi that was also running the app, in addition to passing through the data for the physical faders. The Pi then had a single network connection to the XR18 on stage.
That was an interesting rig too, because it also had a "postage stamp" of an analog board in back as well, that I plugged a phone into for a jukebox and everyone else was welcome to use too, while I shutdown the digital controls and put them in a closet. The XR18 was still in charge, with the analog board going to some of its inputs, with limiters in the XR18 so that they couldn't blow tweeters anymore! I just loaded that scene before I packed up. The analog board could also take a couple of mics in addition to the jukebox phone, so a smaller event could run with just that.
The "guerrilla concerts" always had some sort of "mini-drama" in addition to the live music, and I've also done some full-on theatrical productions with that rig. Things like "The King and I", "Annie Get Your Gun", etc. Those were fun too!
- More characters than we had mics, and more characters than the 16 channel strips in the mixer too, so lots of double-patching and some scene-switching mid-show, while the backstage crew swapped the mics around.
- Sound effects from the same remote-controlled jukebox that runs the canned intro music for the "guerrilla concerts".
- Sometimes terrible mixing position in order to sell more good seats. Do most of the mixing in rehearsal, and only *tweak* live, and not too much because you're hearing it wrong in such a bad position. Trust what you did in rehearsal, despite the acoustics changing with an audience, compared to an empty room.
