PCIe 4 and 3200MHz ECC are readily available on the Ryzen and Threadripper platforms. But if 8 memory channels is a hard requirement, then AMD's answer for you

PCIe 4 and 3200MHz ECC are readily available on the Ryzen and Threadripper platforms. But if 8 memory channels is a hard requirement, then AMD's answer for you is the above mentioned Threadripper Pro in the ThinkStation P620. If you can live with 4 memory channels, consumer Threadripper opens up your options enormously.