Then what's about using 4-8 cheapest motherboards with 64Gb DDR4 and a cheap CPU, and connecting them via PCIE x16 sockets?

And as for DRAM channels, typical cheap motherboard has 2 channels and 4 slots, it should not be super difficult to add 2 more channels.

Don't you need a new CPU to support more channels? It doesn't sound like a cheap, trivial change to upgrade my Ryzen to a Threadripper.