Mac Pro 5,1 - OC Ventura used to work, now it doesn't anymore

Hello everyone,
I just can't understand what went wrong with my Mac Pro 4,1>5,1 (dual cpu, 16 GB RAM, AMD R9 280 3GB flashed, High Sierra) so I try to explain every single step I followed in order to troubleshoot the problem.

1. I've first installed Mac OS Ventura following this tutorial: https://www.youtube.com/watch?v=lU19o8OHvVM. Everything went as expected, I've installed OS in a new Samsung SSD and for 2 days was all good;
2. Since I have a 4K LG monitor and I couldn't make it work at 60 hz, I've done some research and found out HDMI cable doesn't support 60 hz while DisplayPort does, so I managed to buy a mini DP - DP cable. Everything was smooth and fine with my high res and 60 hz Ventura OS;
3. Here's where it gets weird: I'm totally SURE the computer booted up at least for 1 day with this kind of setup, the new cable and so on. The day after, by the way, computer didn't seem to boot in any possible way, there was just the chime and nothing else, no screen, no keyboard seemed to work so no NVRAM reset, nothing at all. I've tried to boot it with the long press and still nothing, changed back my GPU to my GT120 (still mini DP - DP cable) and still nothing. In this phase I'm sure I managed to have the keyboard working somehow, so I've done a NVRAM reset (single reset, double reset, triple reset, everything was suggested online lol) but still nothing on the screen, I've even tried removing all the hdd and ssds and having just the High Sierra one, the answer was nope;
4. Searching here and there I've found that the issue was cable related, as Mac OS doesn't initialize with a Display Port configuration > 1.1 (so neither 1.2/1.4 are supported - my screen doesn't have 1.1); as absurd as it seems, the system booted up normally with HDMI and the AMD card, but I've somehow lost my boot screen (even if the GPU is flashed). This was pretty simple to fix, as NVRAM reset re-enables SIP, so with the GT120 again and an old DVI monitor I was able to get to the recovery, disable SIP and boom, I've got my boot screen back with my AMD R9 280;
5. From now on, there's no way to get into Ventura OS again. Every time I boot into EFI Boot and select the OS, the infamous prohibited sign comes right on, with the link to the startup section on the Apple website. The strangest thing is I can't even reinstall Ventura: I've tried to boot it from the USB with which I've installed it, and I've even tried to re-build the USB with OC and try again, and the result is always the same prohibited sign, as if the Mac isn't able to boot into OpenCore anymore.

I've tried everything I could and still nothing happened since then. Help will be super appreciated.