It might be fun to have so many machines, but in reality it's simpler and cheaper to virtualize everything on two or three powerful hosts. Considering that you're using a soundproof rack already you might as well go with used rack servers with lots of memory and compute. Those also come with goodies like BMCs, dual PSUs, and ECC.
Personally I have two Xeon rack servers running in a Proxmox cluster with a SBC Qdevice. It has more than enough memory and compute for my needs and it also serves as my virtualized router and NAS. The whole setup only takes up 4U of space (servers + switch + modem/qdevice) with a single UPS on the floor, and idle power is around 150W.
We may be optimizing for different needs. For instance, while I was able to get a significant amount of extra height, I didn't have a lot of cabinet depth to work with, which is somewhat limiting for traditional server hardware. There are short-depth options out there, but I also wanted at lease some GPU capability. The integrated GPUs in the SER9s are not top of the line by any means, but they're more than capable for what I want to be working on.