It is true that Nextcloud has a very large ecosystem, but keep in mind that Hubzilla supports advanced features that would greatly benefit your project. The Zot protocol is very mature and has been in development for years… Zot allows a user to easily copy, clone or migrate their user account from one trusted server to another. I’ve personally been using zot for over a year and it works beautifully! Since it is coded for PHP integration is a definite possibility. Just my .02 and thanks for reading. 
edit: I see @andrew from Hubzilla project has shown interest in this kind of integration back in 2017.