You are correct that a reboot will trigger a full rescan. I’m always on the lookout for better sync. I just don’t think it’s out there right now for easy bidirectional sync.
Basically, if you want to set and forget, Syncthing is the best option. If you want more control, you’ll need to look into setting up rsync scripts or similar, which will at least better let you control how often to sync.
As a nerd, I don’t expect my parental control settings to work forever. They’re more there to prevent childish naivete from getting them into trouble, they probably won’t stop dedicated teen horniness. And I won’t even be mad, figuring out how to get around them requires learning more about how technology works.