You mention to not build the website, iOS app, and Android app all at once before getting market fit. Which of the three would you start with to quickly iterate and get feedback?
When you mention that you had to often walk into like a scuba shop to try to get a sign on, were people more receptive if you 'had an app for that' already?
Do Flutter and cover all 3. Okay Flutter Web is pretty bad, but if you are cash strapped and need to move fast, Flutter is the way to go.
I expect more and more startups to embrace Flutter, it is easily currently the best solution on the market (for 99% of app ideas). Yes native will always be better, but if you are a startup, your focus should be building an MVP and getting to product-market-fit as fast as possible at the lowest cost.
Curious, why not React Native? I never heard anything bad about it in Android and iOS (except maybe the resource usage) and at least Microsoft is putting RN as first-tier framework in Windows nowadays.
Ironically, you can't use it for Web. You can't even share components, IME.
Personally though, I've found the DX of Flutter far above RN. I always had random packages break on RN that I had to fix every time, while with Flutter most of what you need is already included in the framework, including a component UI library for Android and iOS.