Something that worked for me:
I tried most of these things, apart from the Windows 11 rebuild. I ran procmon.exe and round SearchIndexer.exe was just constantly reading through HKLM\SOFTWARE\Microsoft\Windows Search\PluginResourceData. I looked in there and there were thousands of entries, compared to much less on a fresh PC.
So I backed up that key, deleted it, restarted the Windows Search Service, and it has just indexed 20,000 items in under a few minutes ! Before it had done 4 files in 2 hours.
I am not re-indexing my Outlook again, as that was what was originally slow.
As always, be careful when deleting or changing things in the registry. But this was the breakthrough for me
Simon
Edit: Annoyingly, I never read the last comment above mine, and it is the same solution. Still, I am happy I worked it out for myself, and it is validated as a fix by Microsoft too. Procmon.exe can be the saviour for many issues.