I don't need a ton of memory so whatever DIMMs are electrically compatible with the CPU will do the job (I imagine that 8 DIMMs of 8GB each is common?). I want it as a development platform for this PCIe 4 peripheral, so that's a hard requirement, and I would like if it resembles a production server as much as possible, so 8 channels at 3200MHz is desirable.

TRX40 with a Threadripper 3960X supports PCIe 4.0 (64 CPU lanes) and 8x ECC UDIMM. This would be a quad channel configuration (2 DIMMs per channel), not 8 channels and DDR4-3200 ECC UDIMM may be hard to find at retail. Threadripper is based on the same silicon as EPYC, though, so it may be close enough for your purpose. ASRock TRX40 Taichi, ASUS Prime TRX40-Pro or maybe ASRock Rack TRX40D8 could be a suitable motherboard.

Threadripper Pro will be even closer to EPYC with 8 memory channels, 128 PCIe 4.0 lanes and RDIMM/LRDIMM support. However, you'd have to wait until September and the Lenovo ThinkStation P620 may be the only choice at time of launch.

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.