Huge thanks to ubahnverleih for his post on this matter.
I have no idea how it works, but this has solved the problem for me (disabling HyperThreading).
After around 10 days of total failures to actually get into MSFS 2024, I can now do so consistently.
Thanks also to JamesGerlach465 - I have amended my workaround so that I do not need to worry about entering the BIOS again to change back.
This community really is wonderful!
I really hope Asobo sort this so that this “workaround” is not necessary any more.