I think you should go with Flutter any day because there is a large variety of widgets inventory for it which can fasten up the development time to at least roll out a minimal viable product out there. Performance of Flutter Apps are also better than React Native apps. And as an added bonus, if in future you want to add a web interface for the same, you can do it using the existing codebase if written in Flutter (dart).
I completely agree with this. Just wanted to add that Dart (the language used by flutter) and the design paradigm (fully functional) can be hard at first. Once you tinker with it you can almost create any prototype in no time.
IMO if you need to build a simple and quick UI for your app, I'd suggest you go for either React Native or Android with Java or Better Yet, Kotlin (It won't take much time for you to get familiar with it if you know Java. See this https://play.kotlinlang.org/koans/overview). For Backend APIs you can use Python with something like Flask