VMs usually have their virtual NICs connected to a bridge interface on the host (like a virtual switch) so they can communicate. Proxmox creates one up by default that is also bridged to the physical NIC you set up for management when you install it, so it just works.
In the router case, you'd likely want this default one to be the 'internal' network and have a separate interface (either physical or VLAN) for the WAN.
I am not perfectly informed, but in my case, OPNsense would need to be the only vm with access to the incoming NIC port, and all other VMs and the router would need to use virtual network interfaces only coming from OPNsense for incoming. The router would be the only device with direct access to the outgoing NIC port. None of that seemed incredibly difficult looking into it, but still, it was the type of recipe I was expecting when I saw "Proxmox scripts".
And of course this means that the Proxmox box as a whole should have similar hardening to a typical web server, with minor tweaks to allow residential traffic on various other standard ports. So that hardening would probably be another script I would like to see (I don't know what all the proxmox scripts in the first section do).
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.
I was looking at a proxmox/(pfsense/opnsense) tutorial the other day. They recommend binding the WAN interface to vmbr1 (or anything other than vmbr0) since VMs are created with their ethernet bridged to vmbr0 by default. This configuration is what most people want so it'll be a little less work setting up networking.
I'm confused by what you mean here? Don't they just use the network like any other computer?
I haven't had to do any special configuration to get my VMs to talk to each other.