> it could be an extremely capable server. If not for the minuscule SSD.

You can get adapters from Apple's custom slot to a standard M.2 form factor, allowing you to use a modern M.2 NVMe drive. As long as you have already installed the firmware update that adds support for NVMe in addition to the AHCI protocol originally used by that machine, there shouldn't be any issues putting in one of today's relatively cheap multi-TB drives. The Mac Pro didn't have the physical space limitations that made some SSD adapters problematic for the laptops.