Ok, I have some news.
After several days testing different things and cleaning my EFI, I FINALLY FOUND THE SOLUTION! It was all about the quirks. Even if it all worked great in Big Sur, for some reason I needed to active a couple more quirks to be able to upgrade to Monterey.
If anyone else has this problem, with that MSU 1130 error or something like that, this is what I did. I don't know if all of this is required, but hey, it's what I did, it solved the issue, and as Monterey is now installed and perfectly working I can't test if some steps are not required, so here's the list of things I did:
- Deleted the APPLE folder inside my EFI folder.
- Enable "AllowRelocationBlock", "ProtectMemoryRegions" and "SyncRuntimePermissions" in the Booter section of my plist.
- Leave all the other quirks exactly as I had them.
- Misc -> Security, I set SecureBootModel to Disabled.
- Reboot the system, and in the picker I chose Clear NVRAM.
- Boot Big Sur. No issues booting. Run the Monterey update again. This time, I had the feeling that it took a little longer than before.
- System reboots, and the update process begin. Everything went smooth, and after some time, Monterey loaded and everything works, wi-fi, bluetooth, graphics acceleration, sound...
This is now marked as solved. Thank you all!