This is just an Idea I would like to share.
Your backend service has really everything any app would need and your support is amazing.
I think introducing an RDMS as an alternative to NoSQl would be a great asset for many apps who need more relationship style than big data. It could be an option to sign up for or a product to choose. This would make you one of the very few backend solutions to provide this capability as many (including firebase) provides only NoSql.
With your structure already in place, I don't think it is major change to add the capability of MySql or another flavor of RDMS
Just a suggestion for future. : )