Minisforum offer a Mini-ITX board with a 16-core Zen4 AMD CPU soldered on for under $400. The AM5 socket version of that same CPU alone is over 500. It uses SO-DIMM DDR5 so might be an interesting option in your case. (Yes, it is a mobile CPU but it has the same amounts of L2/L3 Cache as the AM5 chip, just clocked 300MHz slower)

https://store.minisforum.com/products/minisforum-motherboard