A quick update from my side:
I did an uninstall of Cubase 10 (still keeping my old Cubase 9.5). I made it in steps. First I run the 10.0.30 setup to uninstall each component. Some failed but succeeded running uninstall a second time.
Then I downloaded the full 10.0.30 installer from Download Assistant. I unzipped to a folder C:\cubaseinstaller to avoid too long names in the path to the files, which might be a problem from a download folder.
The installation went fine, but when starting Cubase I got the same kind of error message again. This time for the file AppData\Roaming\Steinberg\Content\VST Sound\FCP_SMT_146_GA_Studio_Kit_Eco.vstsound
Now I saw that this file actually was installed in C:\ProgramData\Steinberg\Content\Groove Agent\VST Sound so I clicked on Locate and pointed out the file. (I now see that a shortcut was created in the AppData folder pointing to the file in ProgramData).
After this Cubase started as it should, and it looks like it works.
However I see that FCP_SMT_152_GASE_Acoustic_Studio_Kit.vstsound is not among the installed files, so I’m not sure that the installation for Groove Agent is complete. The files I find in C:\ProgramData\Steinberg\Content\Groove Agent\VST Sound are:
I have asked the support if this is correct and wait for an answer.
When it comes to other things than Groove Agent:
Except for the ones existing in Cubase, I only have one other VSTI installed on my PC, and that one still turned up without problem in Cubase 10. And the 8 VSTI included in Cubase are also there. The only thing I was missing was that my project templates in Cubase 9.5 were not copied to Cubase 10, but I could copy the files manually.