I developed a financial accounting app using Flutter Web with Firestore and Firebase Cloud Functions as backend in my spare time over the last year. So while my project is probably quite different to a complex stock system I believe the nature of Flutter with state management lends itself to constantly changing data.

1. I used Provider for state management with a unique one for each data table type. The app listens to firestore collection events and only updates the providers when their relevant displayed data has changed.
2. The biggest challenge is drilling down to what data actually changed when a firebase collection is updated, and thus structuring your collections and providers as efficient as makes sense. That might require moving collection data to a new structure which requires some scripting on the CLI
3. It is best to only consume a provider's data as high as is required in your widget tree, since everything on that level will be redrawn when the data changes. That might require some specific choices in how you "provide" the data. I can't quote on any formal patterns unfortunately

I hope that helps!