I’d say leave out the 250GB and install proxmox on a mirror on the 2x 512GB drives, it barely uses any space, just remember to set the root size to like 32GB. Then you have redundancy for the boot partition too.
If using ZFS for the SSD mirror you could use it for the HDD too, but if you want the best performance something other than ZFS may be better.
The most generic simple setup IMO is a Debian minimal install with Docker, and use Komodo if you want a webUI to manage Docker stacks.