How did you like Flutter for this? I've been playing with it some, and overall I like it a lot. I think they've made a bunch of smart choices, and the developer experience is great. But library support ended up being a huge problem for what I was trying to build. E.g., the official Flutter webview [1] is still marked as "Developers Preview" and says "it is not recommended to rely on webview keyboard in production apps". And I was also very disappointed with the mapping library support.
I mostly really like Flutter. I haven't done a huge amount of UI development work in my life, but of the things I have used, I have found Flutter to be far and away the easiest to quickly express the UI you want to build and just build it.
I think its killer features are the cross-platform support by default and the hot-reload—I found the hot-reload functionality so invaluable that I ended up jerry-rigging hot-reload into my current product at work.
You're right that it is definitely still rough in some places though. The libraries on pub.dev can be very hit-or-miss, and I definitely wasted some time trying various libraries from there before either giving up or rolling my functionality.
I too explored a mapping project using it a year or two ago now and ended up having to essentially create my own map view using mapbox APIs (unfortunately I don't have code that I can share from that project). That said, I find Flutter is at least relatively easy enough to implement things that you can't find from the ecosystem.
[1] https://pub.dev/packages/webview_flutter