With AWS Lambda, Python, AWS Cognito, Postgres and ReactJS I can build large scale complete applications.
I think I'm vastly more productive today than I was then. That's partly a function of being a much better programmer in general, partly about now having developed expertise with my chosen technology set.
There's no way I'd go back to something as bare bones as Bottle or Flask any more. If I had to use a Python framework I'd look at a batteries included system like Django. As I say however, I've settled on Lambda, Cognito, Python, Postgres and ReactJS. I've done the hard yards... years of learning and many thousands of lines of code written.
>> Need to subclass and override much of it to finally have something usable.
This is really not something you should be spending your minimal available programming time doing.
After having done it a few times I personally don't find Flask auth very time consuming to implement.
But I'll admit I really like Flask. I find it just the right approach between something simple and something that can be much more complex with extensions. I don't like all of the extension (WTF Forms for instance can do without and SQL Alchemy usually is better independent of Flask IMOP) but these don't need to be used. I've stuck with Flask as much as possible over the years and that alone has led to a huge productivity increase for me because of familiarity.