I think that you should reconsider your approach in order to solve this situation, like this:
Re-create the PowerAutomate flow with a trigger from PowerApps (and send the Record1 data as parameter via JSON). Please see this topic to understand how.
https://powerusers.microsoft.com/t5/Building-Power-Apps/Passing-the-PowerApps-Collection-to-MS-Flow/m-p/299601#M86216
Inside this flow you can write to CDS - Record A, and also its child records 'Record B 1, 2,...,n'.
At the end, respond from PowerAutomate to PowerApps with one parameter that confirm that the flow is finished.
The flow should be triggered from PowerApps app with a code like this Set(flowFinished,flowToUpdateData.RUN(RecordA))
In Screen 2 you will need a Timer with OnTimerEnd property set to check the value of flowFinished and a full-screen label with its Visible property set up with the result of previous timer check and Text property something like "Loading ...".
Hope it help my idea !