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.
I’ve had a few of those little gimmick penis candies and they are terrible. I mean, it’s just some trash molded into a penis to be funny. Where are the gourmet candy penises?