Thanks for your reply! I tried it, realized I still had the same panic at boot about the incorrect number of threads.
In the end I found out the solution was not to do some _STA to _XTA renames for SCKU, and not to put the processors directly inside \SB.
The one and only solution is to renumber the processors, using the script that I had found 3 years ago on ox86.net (fixproc.rb, attached).
The resulting change is this: https://www.dropbox.com/s/925h9izc6lyaroc/Screenshot 2021-06-28 at 14.06.41.png?dl=0
So my question is now:
How can I use OpenCore ACPI patching to turn
Processor (C000, 0x80, 0x00000410, 0x06)
into
Processor (C000, 0x00, 0x00000410, 0x06)
Once I have understood for one processor, I will generate a script to do all the other ones!
fixproc.rb.zip