You can skip the italicized section: I’m a little late upgrading from 3.5 to 3.5.10, but just did it. I rebooted. However, I’m not getting sounds (but certainly did on 3.5). Coincidentally there’s been some Windows updates recently which may be a factor.
Halion SE seems to be ok – I see it in Play mode.
Never mind – solved. In case someone else has a problem, I finally solved it using this part of the YouTube video on sounds problems (Troubleshooting Playback Issues in Dorico | Support - YouTube Resetting HALion Sonic SE database, by deleting the VSTAudioEngine_64 folders(there were three of them – blank, 2 and 3, and I got rid of all three).