I found this article Sync Relational Databases to NoSQL like MongoDB and was sad to see only MongoDB on the dropdown list: 
Is it possible to sync a relational database to RethinkDB with SymmetricDS or any other similar tool?
I found this article Sync Relational Databases to NoSQL like MongoDB and was sad to see only MongoDB on the dropdown list: 
Is it possible to sync a relational database to RethinkDB with SymmetricDS or any other similar tool?
It should be, but you’d need to add yourself support for it. The process is quite straightforward once you understood how it works for other database managers (mostly copy/paste with some modifications and lots of tryouts)