It appears all the NICs are in the same IOMMU group...which is bad. However, they do say they support SR-IOV. Now how one goes about setting that up on Proxmox...going to have to read the documentation.
I think I'm onto something here. I yeeted all of the primary:x devices, and just defined one interface. Now my config looks like this:
iface lo inet loopback
iface primary inet manual
iface vmbr0 inet static
I was going off the thought that I needed to define an IP on the "primary" interface, so I went with static. It turns out, to use this as a bridged device, I need to use manual on this interface instead. Then define the first usable address in the vmbr0 interface. From there, I just add a network device to the CT/VM, and use the next available IP that was assigned to me. So far, it seems to be working!!