@kensonlatchmansing Sorry it took a while to find some time to try and test the scenario you've described above.
Unfortunately, I also could not find a way to use the pool user of an App Service Indicium in combination with a SSRS server. A Power BI server could probably make it work when using a managed identity on the App Service but I did not test this since the Thinkwise Platform currently does not support Power BI as a report type.
Setting the MetaSourceConnection:Username/Userpassword options instead also will not work since the code only seems to override the credentials on each dataset in the report and not the credentials used to authenticate with the SSRS server itself.
So, for the described scenario, the platform would probably need to support configuring a location + credentials of a SSRS server in the model itself. Like how file storage locations etc. can be provided.
This is just a thought since I don't know how complicated the reports are but if they only differ in styling between your customers it might be a lot easier to make those styling options part of the application and send them to the report as parameters. Then you could just include the rdl file of the report using the SSRS (Local) type and allow Indicium to contact the data source directly (assuming all the data comes from the application database).