Flutter is brilliant, and probably the way to go, for some apps. A great example of a successful Flutter app is the Sonos app, relatively simple and responsive ui talking to an api. No “rich” or user supplied content. Although they haven’t replaced their desktop app with the flutter app, my guess is that it is the plan.
I think Flutter is brilliant for that type of IOT app, banking/crypto/utilities also the sort that would work well. I don’t think it’s best placed if you want to start using WebViews for some content, when I was last looking at it a year ago there were serious problems.
If you are building a social media app, either Native or React Native (or a combination) is probably the best way to go. You are aiming high so need the best performance and ux.
If you are building a b2b SAAS app I would probably reach for something like Ionic/Capacitor. You can share almost all you codebase with your web/PWA/desktop (electron) apps and can achieve it all with a tiny team. If you need a little more native functionality then adding in NativeScript to a Capacitor works very well.
For an e-commerce app I would probably go Iconic/Capacitor too, the Amazon app for example is mostly webviews.
Productivity apps are much more difficult to recommend for as you probably need large native components.