ASUS gets asked about BIOS updates for X370 to support Ryzen 5000 similar to ASRock, they reply by saying that there is no official support not even from ASRock, when presented with
ASRock First to Support AMD Ryzen 5000 CPUs on X370 Chipsets | Tom's Hardware
The ASRock X370 Gaming-ITX/ac Motherboard Review
ASUS & Gigabyte Roll Out AMD Ryzen 5000 & Ryzen 7 5800X3D CPU Support For X370, B350 & A320 Motherboards