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.