Yes your motherboard will most likely end up supporting the Ryzen 3000 series. Yes you will need a UEFI/BIOS update which may not be available yet. I've had reassurances from my motherboard maker (ASRock) that my AB350 PRO4 will get Ryzen 3000 support but I am still waiting for the necessary UEFI/BIOS update.
There is a chance your ASUS Prime B450M-A motherboard will end up PCIe 4.0 to the first slot, but even if it doesn't you will still be able to use PCIe 3.0 parts like current generation video cards and NVMe drives. Remember PCIe is backwards compatible with previous versions of PCIe. PCIe 4.0 will not initially be important for consumer graphics cards but will have an impact on the performance of high quality NVMe drives.