VMs already use virtual network interfaces, which are by default bridged to `vmbr0`, a bridge that proxmox creates by default which is also bridged to the hardware NIC. For your use case, you simply want to create a second bridge, e.g. `vmbr1`, which is not bridged to the hardware NIC. You would then assign two virtual NICs to opnsense, one on each bridge (WAN and LAN, essentially) and then choose `vmbr1` as the bridge each time you create an "internal" service behind opnsense.
Since selecting the bridge for a service's NIC is part of setting up each service, the only thing such a "glue script" would be doing is creating the `vmbr1` bridge. That's already a one-liner.
Since selecting the bridge for a service's NIC is part of setting up each service, the only thing such a "glue script" would be doing is creating the `vmbr1` bridge. That's already a one-liner.