On ".net Framework" it is a supported scenario that assemblies with datasources and/or functions are loaded into the standalone ReportDesigner. This assemblies are used while designing and later also in the ReportViewer while executing the report. Feature described here https://docs.telerik.com/reporting/standalone-report-designer-extending-configuration
Now that .net 6 is release by MS the same feature is needed there. The problem is that the "standalone ReportDesigner" is targeting ."net framework" which means it can only load assemblies targeting ".net framework" or up to ".net standard 2.0". But that is not sufficient because on ".net standard 2.0" only EntityFrameworkCore3.1 is supported and all newer version (EF Core 6.0) are not. See here https://docs.telerik.com/reporting/knowledge-base/use-reporting-in-dotnetcore-with-entity-framework-core
So in summary:
It`s not possible to use EF Core 6.0 (and upwards) DataSources in the given "standalone ReportDesigner" because it is targeting .net framework. It`s critical to be able to use the newest EF Core versions as DataSource to not be blocked from upgrading a ".net framework" application to .net 6 where the described feature/scenario is used. I need to use the "standalone ReportDesigner" because it gets shipped with the application to edit the reports individually by the enduser. The "VS ReportDesinger" is going to get the upgrade but this would only be usable as a developer before shipping the application.
Upgrade the "standalone ReportDesigner" to target .net 6.