React, Flutter Web and SEO

Hello SEO community.

I am currently starting working on a startup with a couple of friends. For the purposes of our work, we need to develop a mobile app as well as a corresponding website. Regarding the app, we have worked on flutter, with quite satisfying results. However, we are wondering, since SEO will be of crucial importance to our service, whether we should stick with Flutter (Web) or React or should move to other frameworks/plain HTML5/CSS etc.

So, what's your experience/opinion on this?

Thanks! :)