Thanks again piero!
As it happens, I eventually realised that the reason I got the message in red saying “Only official released binaries are allowed to be flashed(RECOVERY)” or “…(VBMETA)” was that I had dropped into KG/RMM State “Prenormal” - a state which apparently is cleared only by the RLC service on the device making an inquiry to a Samsung server to make sure it is “clean”, and which it can make you wait 7 days to do so. In my case the problem was simply that I had not connected over wi-fi since unlocking the bootloader - as soon as I did so the KG state went to Normal and I could flash the /e/ bootloader successfully. The Know your hardware - Samsung page mentioned in this thread really helped (as did the xda-developers pages linked from there and from in this thread).
The video link you gave *Watch on YouTube Watch on Odysee was also really helpful (especially with how to do the button presses & co-ordinated USB plug-in).
Thanks again for amazing support!
Bill.