From the Ruby side Sinatra is just as fast as flask to get something going. Having supported all three ( Rails, flask, Sinatra ) if it's going to be an API. I usually start with Sinatra.

Agreed. Flask or Aiohttp + templating engine of choice + flask-login or aiohttp-security is extremely fast and simple.

Pyramid - https://trypyramid.com/ - lightweight like flask but with that little extra that makes life easier (authentication, sqlalchemy integration). Not as heavy/opinionated as django. In fact, would argue, not opinionated at all.. just the right balance.

You left out persistence, which is the most annoying bit of setting up a Flask project, imo.