Hello, greeting from Brazil.
As required in your project I have dedicated and expert team of flutter and blockchain developers who have experience in tasks related to blockchain. We would also create the design of the app (after completed, you would be able to publish it in Android and IOS).
In order to start working, we just need some more informations about the app functionalities (also, will you only work with btc, or you would like to have other altcoins in the app like etherium)