Thank you all for this. 

My issue is that I already know a bit of flutter but find it clunky and never really liked it. Also, the rumours are that Google has abandoned it and the flutter team is a little adrift. 

React may be a tested and proven way to get something done and the ecosystem seems capable to solve any problem. 
Kotlin is a whole different ball game, because it locks me to android (has is pros and cons) but ultimately if the aim is to release my own products, kotlin seems like and overkill for now. 

If what I need to to prototype faster, flutter is too clunky and hard and has a bad outlook. React seems like the sensible option.

Agree?  

 I always thought React was better than Flutter. But it is also a bit slower. But it's def. great for prototypes.