To add some additional context to this - I'm planning on using an OPNSense VM to act as the firewall between my virtual subnets and the LAN.
Because Proxmox DHCP is using the .1 address for my subnets, I have the OPNSense available as .2.
I'd like the default route to point to the .2, but changing the gateway in the subnet config seems to also change the DHCP IP. I've already got Proxmox talking to PowerDNS so I want to keep using its DHCP & IPAM solution (I'm also not sure I'll be able to do all of the things I want to do with Unbound DNS, but PowerDNS should be able to do it).