Here the problem disappeared after fiddling with the siri settings in each app.

I read in the last edition of my computer magazine this tipp: Goto Settings, than „Siri and Search“ In the setting of every app turn off „Show content in search“ (maybe it’s called something different, because having a German phone here „Inhalt in Suche anzeigen“) Than restart phone

One day later at 16.6GB

Update to iOS 15.5 reduced System Data down to 10.5GB

..

Having the same problem and tried everything, except of reinstalling the phone. What brought some relief, was connecting the phone to the Mac via cable. It appears now, that my pictures are consuming now much less space and system data went up. In sum, it was nevertheless significantly less. Like from 2GB free to now 36GB free space. The idea to delete Safari history/cache is from my point of view total mockery. How can Safari ever consume data in the range of 50GB and more? Don't get me wrong, but if I read something from "delete your browser cache" I stop reading.