I was a beginner with React and NextJS too. I opted for it because it seemed fairly well supported. It has been a learning curve but it is feature rich and VERY well supported and documented. I would recommend using it (not to mention, NextUI is beautiful and great examples on their website for getting started).

As for using it purely for a backend, it depends on your requirements. The most mine does backend-wise is I have a couple of api routes for getting the session token out of a cookie. I use Scala+AKKA-http as my server (with Hibernate + mySQL), meaning the database is not stored on the web side at all. If your system is quite complex (like mine), I would recommend not using NextJS for the server side too, and look at other languages that are designed to operate as a more complex backend webserver.