i registered here just to report what i figured out about this mess, with thanks to others for all the collective trouble-shooting. i've tried every suggestion on this thread and others, to deal with panics and inability of the mini to sleep. i even installed the Big Sur beta on a separate container on my internal ssd. (did not work.) i've been talking to Apple support for many weeks.
i use an APC uninterruptible power supply, which most of my system uses for protected power. in desperation, i started swapping all the power plugs and outlets on the UPS. (other people have reported success with switching to alternative power outlets, which gave me the idea.) surprisingly, a particular combination of plugs and outlets allowed my system to sleep!
at that point, i figured that my UPS might be faulty, or was in conflict with the mini. or that there was some issue about what external devices shut down first, or that some kind of power surges were restarting devices and interrupting the sleep process.
here's the kicker. like an idiot, i decided to install the latest security update. what a glutton for punishment. i thought it would be okay, since i "knew" the problem was with my incoming voltage. but after i did the update, my mini could no longer sleep. again! i consider this to be (another) proof that the security updates cause the issues we are having.
i shut my computer down, turned off the UPS using the power button, waited a minute, and turned it back on. my computer sleeps again!
the lesson here, at least for me, is that the power management system of the mini is very finicky. probably because of the T2 chip. the security updates further mess up the power management of external devices, causing them to restart just as they begin to go to sleep. playing around with the power sources for my mini and my external devices allowed me to balance things out somehow, and get my mini functioning again. but only until the next security update....
your mileage may vary. i hope this helps some of you