other8026
"One important thing to remember is that you cannot flash while in incognito mode."
YOU GOT IT !!!! Pure dead brilliant, mate !
Thanks a million, other8026.
II created a Google account, connected in the browser under this identity, and lo and behold : the Web Installer finally accepted to flash Graphene OS on my Pixel 7a.
I confess I did struggle with this for the past two days. You're a genius, pal!