Nice project! I just started researching the pieces I need to build something similar.
What kind of PoE hat did you use? I was also thinking of having one of the PIs have also an SSD with bootp/dhcp and let the others boot with PXE.
I had been thinking of building a PIs cluster for a while now and I was looking with interest at the turingpi board project [1] they are taking preorders for the second batch if anyone is interested. But I would like nodes with more than 1GB of RAM so I guess I'll have to wait for that.