I finally got around to writing my UrBackup backup software 
I hope you find it useful, as I’ve tried several btrfs clone programs that never really quite worked how I wanted. I use this to copy all subvolumes and the database (and backup database) to a LUKS encrypted btrfs filesystem that is rotated offsite. I can plug in my old filesystem, and the program then “clones” the changes. Much nicer than the rsync version I was using before.
Github link:
Cheers!