A single 3600X will grossly outperform this cluster (and cost less) with less headaches (you don't have N physical machines) by using KVM to deploy a few virtual machines and using Kubernetes to orchestrate and allocate within those VMs. You'll also have a lot less latency between nodes running in VMs on the same physical host.
Another thing that unfortunately sucks about Raspberry Pis (less with Pi 4, but still mostly applies) is really shitty I/O performance...
I spent a large amount of time over the past summer and fall trying out various ideas to have a "cluster" at home that was both practical and useful. While, the PIs were nice, they never really amounted to much more than a demo. Latency and I/O become real problems for a lot of useful interconnected services and applications.
Honestly, if Ryzen 3000 hadn't come out, for cheaper cluster builds (~300-400) I still think Pis would be a solid choice but... Ryzen 3000 is just so fucking fast with a lot of cores, it's truly hard to beat.
Addendum: to touch on used servers, yes your power bill will go way up, no joke, but for some applications like large storage arrays-- it's hands down the cheapest/easiest route. Search by case, not by processor, it sounds weird but the case is likely the most valuable part of the old server (like ones with 20+ SAS2 slots for $500) or PCI-E slots that GPUs can fit into.