Flask has a stigma for not being that good when building larger apps, but honestly Flask scales really well for that type of use case (coming from a development / maintenance point of view). I have some pretty large Flask apps with dozens of top level dependencies and models spanning across many thousands of lines of code. Even if I don't touch the code base for a few months, it's easy to jump back into it. I'm also…
Not sure how to quantify that "large" but I've worked on some api backend services written in Flask with a quarter million lines of python code and not feeling "outgrowing" Flask. Granted, it took some good design and effort to make all piece s work together. These days, horizontally scale apps is the way to go.