After some more digging and trial and error, I found that disabling the power saver option to put the HDs to sleep and enabling the "Prevent computer from sleeping automatically when the display is off" setting stopped the random reboots.
Also, after running Etrecheck, I noticed that the Data drive did not get deleted when I deleted the volume group; etercheck was still finding all the apps and stuff on there. So, I started from scratch one more time, deleted the volume group to get rid of "Macintosh HD" and "Macintosh HD - Data" then manually deleted the other "Macintosh HD - Data" and reinstalled Big Sur again.
When it came up, I only changed the following (and did not install anything extra for a while to keep culprits limited).
- Disabled the power saver option to put drives to sleep (this is an iMac Pro... I'd obviously not want to do that on my MacBook Pro when I get there)
- By default, the screen saver is set to come on 5 minutes later than the option to turn off the screens so I dropped the screen saver from 20 minutes to 5.
I ran that setup most of last night and all of today with no crashes. Eventually (a couple hours ago), I installed office and other various apps I need for work and, so far, everything appears to still be stable. If that changes, I'll report back. Otherwise, I think it might finally be "fixed".