After years of frustration with slowness, inconsistent behavior and general weirdness, I finally got handheld sync to work fast and reliably.
Instead of using my phone's "file transfer" mode for usb connection, I tried "developer" mode. Works like a champ.
Another tip. If you've had many failed attempts and/or your player software is missing tracks and/or playlists aren't getting updated, try resetting the Android media database. Settings > Apps and notifications > App info > select Show system from three dot menu > scroll down to Media Storage > Storage > Clear Data. Restart the phone and Android will rebuild it. It will take a while. It also resets custom notifications, but oh well.
(This is on a Motorola phone with near stock Android 8. Other phones may call it something different.)