The best tech stack when starting a startup is one you don't have to learn.
There's a million things you learn when starting a company. Don't make yourself learn an entirely new tech stack on top of everything else. This advice means you'll be using whatever you've used in the past which might not be the sexiest or newest technology, but your users won't care. Your users want a working product. Choose the stack that will result in a working product the quickest.
Refactor and migrate to a better stack later, if necessary. (It rarely is)
For me, that meant deploying to Elastic Beanstalk (I know, boring, no one talks about it, but it works!) and using Mongo because I was already comfortable with it. It also meant not using React, at first. This was the right answer for me, but might be the wrong answer for you! Build your app on the technology you know.