In fact, I think I never even connected the phone via USB since I started using Syncthing. Copying/moving to/from shared folder is amazingly both more ergonomic and much faster (I never learned, why moving files to Android device via USB is so insanely slow).
So I really don't know an alternative. It solved pretty much all my phone/PC sync needs. It also enables backuping the important stuff (I don't use Google for that, or course). Dropbox doesn't cut it either. I really don't imagine how would I use my phone w/o Syncthing, it's pretty much essential.
It's all those little convenience features that make KDE Connect a great tool for my needs. It's only the file transfer features of it that don't live up to my needs, but for that we've got SyncThing (and probably tons of other perfectly valid options that I've never tried, too). For continuous file sync / transfer, SyncThing has been just "set it and forget it" simple for me. It just "does the thing" and keeps on doin' it reliably. :)