How to preserve iPhone contents/settings and user prefs when doing a reinstall

I recently updated from Mojave to Catalina so I could manage the music on my iPhone since I foolishly updated the iOS to have Dark Mode, forgetting it would mean I could no longer use it in Mojave.

It went much more smoothly than I'd expected — until a couple of days ago when I updated Safari. Now my WindowServer crashes regularly even if I'm not running Safari. Rather than try to troubleshoot the issue, I'm considering a fresh install since I've got apps and services from the Yosemite era still trying to run. I really, really don't want to do this because this is a production machine with thousands of audio plugins that will all have to be reinstalled and activated, but macOS is not what OS X was — user friendly and graceful — so it looks like my only option if I want a stable machine. Ugh!

I just set up my iPhone with a bunch of playlists and would really like to make sure that the first time I connect my iPhone after the fresh install, all of that will be preserved. Where does Catalina store all of that info? I have a local backup on a hard drive that won't be erased when I do the clean install, but will that preserve the playlists too? When I first connected my iPhone to my Mac after the Catalina "upgrade", it erased all the playlist info on it. I really want to prevent that!

Also, is there a way to preserve my user settings and preferences without reintroducing potential conflicts?

Thanks!