Thanks. I hate bothering people but after having spent the day trying to fix my issue I have to admit I need some support.
Shortly, initially I tried to flash my phone without unlocking the bootloader by mistake (as I thought it was already unlocked) and now it doesn't work anymore.
Today I have been able to unlock it (for real, I mean) and install TWRP but I don't know how to copy a rom in it. (adb gives me "unauthorized")
The only "official" image I found is Chinese and I could live with it for a while but when I try to flash it Miflash 2019.12.6.0 never completes its task.
What am I missing?