Short of using OAuth2, you could write your own app component to tie NodeBB into MySQL. One of the issues is that MySQL is a generic database so the format of your authentication easily won't match in some obvious way, so needs a customer connectors no matter what. Just adding another DB source for authentication would not solve your problem. It would have to be something that connects to your database and has the ability to understand your schema and has the ability to store all necessary additional schema changes there.
Solved Change redirect link for /login when already logged in
When my user is already logged in, and the user visits /login, it redirects to their profile page.
How can I make it redirect to root instead?
I ended up creating an oauth plugin for my app based off of this skeleton:
and instead linked it to https://forum.mysite.com/auth/myapp and it performs the intended functionality.