Thanks for the procedure! I was also disappointed by the lack of OTA upgrade (at least now it’s clear it was a technical issue). Your post motivated me to give it a try and it worked very nicely (well, except that I Can't install open source apps but hopefully that can be easily fixed).
The only thing that was not clear to me was point 2: it took me some time and research until I realised I had to type fastboot flashing unlock on my computer when on the bootloader screen. Maybe you could add this to your original post in case someone else faces the same issue as me?