Thanks again for your assistance and time on this.
Started Mini with same iPad Mini (JRemote 3.36), after some normal play to Player, and This Device, I let sit for ~30 minutes. Switched back to player zone and played normally. Repeated, when I restarted the player zone from the Pad, it did not respond. Went to keyboard and when I turned the monitor on and used the keyboard to go check for other windows (nothing showed), it started playing. Played one album, then stopped and JRemote became sluggish/non responsive when I tried to play another album. Turned on display, and when I started to check other windows (none apparent), it started playing. Ran logs and will email. Will also try again overnight.
Just for completeness, this is the physical Mini from the original issue, but there are a couple of differences in setup: 1) Monitor is HDMI attached rather than a Mini Display port to VGA dongle. I turn this monitor off when not needed for startup/shutdown/etc (most of the time). 2) Output is USB to a Topping D10s for SPDIF conversion, then SPDIF (RCA) to an older Benchmark (early model, no USB input) rather than USB direct to an older Benchmark.
If differences in setup complicate this, I understand and can try rolling back to known good versions (Sierra/MC 24). Also willing to try disabling App Nap, if thats the best solution and still available in recent versions of the OS.