For those having problems with Ryzen + Asus + Windows 10 updates, I will leave my "solution" here. I was almost giving up and selling everything when I decided to reduce the CPU clock from 3.7 to 3.5 and bingo. Windows 20h2 installed without freezing and working well (so far).
I know it's not a real solution (if the CPU is 3.7 Ghz, it should work at 3.7), but better than a useless machine. I just hope Windows 21h1 is not so problematic to update...