When using the SDK, it is simple to just add a new plugin into the my_plugins folder. However, when dealing with larger multi-plugin projects that have their own build system, version control, dependencies and libraries this is not the best solution. One (working) option is to add some code to the CMakeLists.txt in the vst3sdk directory that adds a subdirectory outside the build path of the SDK or to even include the entire SDK as a subdirectory but both versions seem not quite optimal (especially when updating etc.).
Can we maybe learn how this is dealt with at Steinberg for example? Or would there be the possibility of exporting a package through CMake containing the VST SDKs libraries in a future version allowing to look for them with find_package()?
I am not quite sure if I get all the aspects of your use case. But basically what you describe shall already work nicely.
In order to get a better idea you can use the VST3_Project_Generator which you can find inside the VST 3 SDK package. Generate an example plug-in and have a look at the root CMakeLists.txt: