Thank you for this. I have checked/tried all of these before posting.
'no other app can interfere with the process' How do I make sure of this?
The furthest point I had reached was when Vanadium came up with the 'Aw, Snap!..' error. Is amy one able to interpret this message?
Is it relevant that the 6a is now partially updated? On power up it goes into a loop at trying to load the OS but I can return it to the Fastboot screen using the buttons.
Whilst I am obviously keen to explore all these factors, I am still extremely surprised at the comparison with the straightforward experience with the Pixel 3. Is the 6a really so different? If so, in what way?