OK, I think the MacRumors thread may have actually identified the root issue here. Based on the hypothesis discussed there, I used the Edit Pages feature to “hide” the home screens that contained many app folders, leaving only a couple of screens with single apps and a single folder visible. Today, about 12 hours later, with no other changes to my phone usage, my System Memory dropped from 85GB to 20GB, and it seems to be relatively stable. (It’s risen to 21.5GB after a couple of hours, but it’s still well within a reasonable amount.)
I’ll check this over the course of the day, but until the issue is resolved, this may be a good temporary fix for those folks who have many apps on their phone (and are suffering from the System Data bloat problem).