Thanks again Ivan.
I'm still struggling, unfortunately.
Profuse apologies in advance for the length of this posting because of all the screen prints and photographs.
I disabled Fast Boot as you suggested but couldn't disable Security as the only security settings are passwords and I never set any. After, I tried installing Win 10 again with these Rufus Settings:

and received this Rufus error again:

Although you say that it is UEFI compatible, I haven't found any mention of it anywhere. A few days ago, I also contacted ASUS, who couldn't even tell me if it was BIOS Legacy or UEFI. They promised to contact me to confirm, but silence so far.
These are photographs of the only BIOS settings that I have available (sorry in advance that some are a bit fuzzy, especially the last one):





Possibly a really dumb question to you experts, but why does the installation of Win 11 work on GPT partitions but not Win 10 1909 when using the same Rufus settings?
Any further suggestions would be highly appreciated, including for example:
(i) Different Rufus Settings?
(ii) What is "Windows to Go", instead of "Standard Windows Installation"?
(iii) Other BIOS Settings?
(iv) Another completely different method, instead of Rufus?
(v) Using a virtual drive? (Maybe a really stupid suggestion because I know absolutely nothing about them . . . I suppose the obvious question would be, "Can a virtual drive be MBR if the physical drive is GPT?")
(vi) Using a USB connected external hard drive? (Maybe another silly option because again I have no idea if it can work. However, if it does, I would eventually unplug it and dispense with it as soon as I can solve the BSOD problem with later Win 10 versions and Win 11).