I’ve since kind of figured out the issue:
The issue wasn’t with the flashing of the titan ridge, but with the configuration of OpenCore.
There is a known bug (and supposed .plist fix) with OpenCore, which prevents OS above 10.14 to use a second display. I was using Martin Lo’s OpenCore package, with the .plist modification, which is supposed to correct this behaviour, but nothing i did allowed me to have a video signal on the thunderbolt screen (camera speakers and all else workeds...)
Long story short, when i booted in macOS 10.14 without OpenCore, I was shocked to see my apple Cinema Display outputting a video signal!
Updating my system to Big Sur with the OCLP (open Core Legacy patcher) allowed me to have a video signal on my apple thunderbolt screen “out of the box” (no further .plist modifications)
BUT, in this configuration I need to unplug and replug my thunderbolt screen after startup (operation not needed when booting in macOS 10.14)
Out of curiosity I tried reverting to Martin Lo’s Open Core package and modified once againg the .plist to correct the "no second screen video output": impossible to get any video signal on the thunderbolt screen.
TLDR:
If you want to use a Thunderbolt Apple Cinema Display with the titan ridge on an os above 10.14, use the Open Core Legacy Patcher