Hi,
this would be a tricky feature as you can select whatever subset of tracks (only even just single track) to send to the device.
And currently the granularity of the pre-configured sync-list is not "per track entity", but rather based on entities like whole playlists, collection's albums, artists, folders. etc.
See: https://www.mediamonkey.com/wiki/WebHelp:Exporting_Tracks/5.0#Sync_List_.28Library_--.3E_Device.29

I wonder why you even enabled the [x] Delete other files and playlists... checkbox? Isn't disabling the option a solution for your issue ?

When in the music hierarchy, if an album or playlist is selected, the right click menu gives an option to send the selected files to a connected USB device, which it does successfully, but it does not add these files to the sync library for the USB device. Later when choosing to manually sync other files, these earlier files are selected for deletion. Is it possible to change the default behaviour to add these files to the sync library?