OK, I think I have an iOS 26 workaround. Based on the hypothesis discussed here, 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. (I can still access those apps via the App Library.) 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 (hopefully in iOS 27), this may be a good temporary fix.