I don't know anything about mobile development and not a lot about frontend.
I'd do the backend with Django and Postgresql. You can't go wrong with Postgresql, it's an industrial strength database. Nothing wrong with MariaDB (or Mysql for that matter), either, I just prefer Postgres.
Writing your backend in Django will allow you to get the basic functionality working in an afternoon. Don't worry too much about getting all fancy with the user models at least to begin with, or any "clever" stuff. Don't worry about doing it at Google scale until you're half way towards Google traffic.
You can use generic views in Django to rough out a web UI for it, then make it as fancy as you like with Flutter.
Comments
I don't know anything about mobile development and not a lot about frontend.
I'd do the backend with Django and Postgresql. You can't go wrong with Postgresql, it's an industrial strength database. Nothing wrong with MariaDB (or Mysql for that matter), either, I just prefer Postgres.
Writing your backend in Django will allow you to get the basic functionality working in an afternoon. Don't worry too much about getting all fancy with the user models at least to begin with, or any "clever" stuff. Don't worry about doing it at Google scale until you're half way towards Google traffic.
You can use generic views in Django to rough out a web UI for it, then make it as fancy as you like with Flutter.