Is Scale out deployment for SSRS needed in an Availability group?
Configuring a Scale-Out SSRS instance on Always On is fairly straightforward. You follow the normal Scale-Out deployment steps but when you connect your first and second SSRS instances to the ReportServer database you simply point to the Always On listener.

There is a great blog article here that goes through the steps from start to finish.

You do not specify applicationintent = readonly in the connection string because you want both SSRS instances to connect to the primary ReportServer database. Use that connection string option in the connection strings for your report data sources (if they are in an AG) to redirect report workload to the secondary replica.

Lastly, using the Virtual Account is likely causing your issues as it won't have access to the RS databases. Use a domain service account instead.