this ain't my first rodeo, brother. Did I mention 30 years?
Though, to be completely honest, no, I haven't booted into safe mode, as there is little else can be done on the mac in that mode, and as it is, the random mounts and unmounts are...Erm...random, and can occur multiple times a day, or not happen for a few days. This has not occurred in high sierra, the little time I dabbled in 10.14, or big sur.
It is on a 'clean' system, ie not upgraded from an earlier one.
If it were only me, I would assume something I had running, but as I say, it's pretty clean with no system-level malfeasance. As you correctly point out, a guest/new user account should put any user-level additions to bed, but they do not. It's a system-level flaw. NOTHING should be forcing an unmounted sd card to mount without user input, *nothing*, and that's forgetting the forced-unclean-ejecting of media.
It isn't just me, and it isn't a user-issue.