I'd advise getting other users to use it for free first before you dive into any paperwork. Basically, make sure you're solving a problem worth money for others first.
Dart is something very similar to Java, not much of a bet to jump into it. Moreover, it has type soundness, something Typescript cannot achieve.
I'm not so sure it would be easy to continue RN core development either. From what I see even just upgrading the JS engine from an ancient version was a major hassle for the core team.
In RN there's simply more moving parts. You work with 3 package managers at the same time, possibly even 3 languages. At least at this stage Flutter just seems overall more productive for the developer.
The last time I was using React Native even text input was broken on Android. It made any app using it basically unusable. Android profiler didn't work either, opening it crashed the runtime.
Flutter has its bugs but they're certainly not that fundamental issues.
I don't have any first hand experience with such a situation myself, but perhaps studying some successful cases could get you to the next step: https://www.indiehackers.com/interview/how-charging-money-fo...