Thank you for your insights and suggestions. Very glad I didn't buy anything yet. There appear to be a number of PCIe options for using both SATA or NVMe, but I've decided to get a couple of smaller, cheap 2.5 SATA SSDs just for the OS.
The plan is to just have Proxmox installed RAID1 on the two SSDs for redundancy, then the real data array is 4x12tb HGST in some ZFS configuration. Does this seem sane?