Okay, I worked out the solution to my own issue…
Apparently, once the networking code tries to treat the RAID adapter as a network card and assign an IP address to it, something breaks and either the switch is shutting the port down due to traffic weirdness, or proxmox is doing something similar.
If I edit /etc/network/interfaces to enp4s0f0 before shutting down and adding the RAID card then it works fine.
Okay, I will likely be back tomorrow once I come across my next hurdle… 