Your approach sounds good. This command starts VS for me with the ability to debug Slicer and SlicerRT:
..\S5D\Slicer-build\Slicer.exe --VisualStudio --launcher-no-splash --launcher-additional-settings ./SlicerRT_D/inner-build/AdditionalLauncherSettings.ini c:\d\_Extensions\SlicerRT_D\inner-build\SlicerRT.sln
Then when the solution opens, make sure that you select a project other than ALL_BUILD as startup, it can be any of them (qSlicer… or vtkMRML…). Then, in the project settings you need to set the command to be the SlicerApp-real.exe, such as c:\d\S5D\Slicer-build\bin\Debug\SlicerApp-real.exe. Then when you start the debugger, the breakpoints in Slicer and SlicerRT (and CTK etc) should be hit.
Of course what @pieper suggests with SlicerWithSlicerRT.exe should work too, but what I have wrote above is how I have been doing it forever and it still works.