Your stack is as good as any. Do yourself a favour if you're building a business... Don't look at it as an engineer. If you're proficient in python, use Flask and Celery. If you're proficient in Ruby use Rails/Sinatra and Sidekiq. These tools are battle-tested, solid. You can build a business around them easily. If you ever hit any limits, then and only then look around for more.

My advice to you is to master these tools instead of learning a new language/stack/tool.

I see ppl talking about Elixir, Erlang, Phoenix, etc. It doesn't matter, what matters if for you to deliver. Python is an excellent choice, the tools, you're talking about are solid... the only thing that matters now is for you to deliver.