I thought I covered that
"Disabled my AV and tried the Windows 11 update again."
Actually, before the last attempt I completely removed the AV (Windows Defender always disabled..), no anti-malware installed.
There seems to be a lot of speculation about 0x80004005 around but little in the way...
Making a very long story a little shorter.......
My system:
Gigabyte B450/16GB/512GB Samsung 970 Pro as C Drive (having run the Health checked and cirmed OK for Windows 11).
Tried the Windows 11 install from Update page (download and install).
About 40 minutes later, back in my familiar...