Stereo Tool

Are you sure that the inputs of ST are unset, e.g. put as stereo_tool_fm or something instead of what actual audio device you intend to target? That sounds like a conflict and i've had them in both jackd and pipewire.
You can do the same thing with the outputs too, set them to loopback or something unused. In the beta version with dual alsa and jack mode you can test further a combination which succeeds.

I wouldn't recommend systemd for user facing things, that thing is a mess. Raysession, carla or heck even plain shortcuts work better.

Thank you, Hitman. I am still trying to figure out how I will want all this stuff to launch and hit the ground running whenever the machine reboots.

At this time, under Ubuntu Studio, I have two instances of ST JACK playing very well, and seemingly stable. In the world of broadcast, "stable" means years. I can't test that, of course, but if I get a solid week of no-break then I'll move on to the exciting and challenging world of launching.

I don't know as much about Linux audio as I wish I did. I thought Carla was a plugin host. I don't see how that would be of use in my application . . . unless we count ST as a plugin. I reckon I'll see if I can compile raysession and play with it.

Ugh. Linux:

~/Downloads/RaySession-master$ make
make[1]: Entering directory '/home/kpov_tech/Downloads/RaySession-master/HoustonPatchbay'
make[1]: *** No targets specified and no makefile found.  Stop.
make[1]: Leaving directory '/home/kpov_tech/Downloads/RaySession-master/HoustonPatchbay'
make: *** [Makefile:35: PATCHBAY] Error 2
$

For it to hit the ground running at reboot it would also need some sort of restoration of patchbay, just so you note, it might be called differently though. Carla calls it Project to sound more fancy.
And yeah with proper broadcasting equipment there still are people out there using Windows XP for that reason, hehe.

That program should be available in the Universe repository in ubuntu, enable it with 'sudo add-apt-repository universe'

Thank you, everyone, for the help.

"sudo add-apt-repository universe"

Right on, duh, yep I have raysession up and running, sweet!

Update for all the fans of this Topic.

Under the setup I have now: a Beelink SER with Ubuntu Studio 22.04, two instances of ST JACKs, each sharing one cheap-o USB Capture card (Behringer UCA222), and each with its own Playback card (the UCA222 and one of these: [url]https://www.amazon.com/FEMORO-Adapter-E ... r=8-20/url]) -- I'm here to report that so far, ALSA/JACK seems to be dealing with these two different devices which are most certainly not running on the same clock, without any ticks of pops, no errors popping up in syslog, and QJackCtl's Message pane is free of errors.

                      /-->  STEREOTOOL_1 ------> PLAYBACK (CARD 1) --> FM TRANSMITTER
CAPTURE (CARD 0)--->>
                      \-->  STEREOTOOL_0 ------> PLAYBACK (CARD 0) --> LIVE STREAM

STEREOTOOL_1 provides aggressive LOUD processing for FM broadcast.
STEREOTOOL_0 provides light processing for live stream and live shows archive recordings.

Linking two disparate sound cards to JACK seems an invitation for clock drift to break things, but so far it's been clean.

Thank you all for the help here. I'll keep this running and if it seems solid for radio broadcast then I'll move on getting it all to load and hit the ground running on reboot.