Yes, thanks @benwiggy , but to be precise again, for Intel based Macs the path there is without the arm64, so just
~/Library/Preferences/Dorico 6 AudioEngine/Dorico 6 AudioEngine VST3 Cache
Thank you so much, Ulf, it is now working perfectly as far as I can tell! Much gratitude to you for taking the time to work and re-work this. I assume that as more plugins are added (!) I should periodically revisit this.
You are welcome @tonycore .
And yes, you can revise the list at any time you like.
It might be useful to state the location for the XML file on Windows OS here, as well. Unless it’s all in the Registry, or something… 
On Windows the vst3ignorelist.xml belongs into
%APPDATA%\Steinberg\Dorico 6 AudioEngine_64\Dorico 6 AudioEngine VST3 Cache
And there is another specialty on Windows, and that is the paths delimiters. Normally Windows uses backslashes ‘\’ as delimiters but in case of the plug-in paths in the ignorelist for Windows you have to make all delimiters forward slashes ‘/’
The VSTAudioEngine will handle the delimiters by itself and convert internally again.