This wasn't so much about flask, but about WSGI.
I am using neither django nor flask now.
But... how else am I going to build my own back end?
PHP?
To be fair: flask is quick and easy, especially when you use all those extra libraries.
Since I am building something unique though, with a blockchain in the background, I have to build a lot of stuff from scratch, so I chose aiohttp and ASGI and let some variant of gunicorn pop up different workers.