Don't get me wrong, I like and use Next. But from what I can understand, it's just not as complete as Django, Rails, etc. I think the fact that template is using Firebase for its storage says a lot.

Next itself does not help you with Firebase or databases or any kind of persistent storage. It's entirely left up to you. That's a huge difference from Django, Rails, etc. The same is true of auth and many other things.

The nextjs one uses a third party (bigcommerce) as the back end.

Firebase is also a third party backend.

Any recommendations on one where you can own the backend yourself, like Django? Without composing all the pieces manually?

I'm currently running a Django backend, Vue frontend, and think Django is a fine tool -- but occasionally look for a nice cohesive Django alternative in the JS world.