ThinkPad W530 fits the bill quite well. Broken screen puts it at around $100-150, $300 gets you fully working one. Less tyhan 4 CM4 modules, and what about Turing Pi 2 case? power supply?

>Scaling horizontally presents entirely new classes of challenges that aren't experienced while scaling vertically.

Pay more for challenges? If you really want to fight with bad scaling just run a VM.