Thanks for the reply.
I started last night installing that patched version of catalina. But then in the first reboot it goes to a black screen again.
So this morning I went and turned it on:
It turns on and kept trying to conclude the installation. But after that when it goes for another reboot:
It goes into the problematic behaviour of black screen, sleep light on, fans on, but no chime and no progress.
Funny thing is if I wait another 20 minutes or so, I can have another chance of it turning on and behaving "normally".
I´m not even sure if this is the gpu problem. Tried changing the ram from 16gb to the original 4gb but the same behaviour is present. Did an apple hardware test and showed no problems..