[gerdesj]

No it isn't basic and common (it is for me but perhaps not for you and certainly not for most people)

OK, so you want to virtualise a router and firewall. That's fine. I have deployed roughly 200 pfSense firewall/routers as VMs and physical boxes and OPNSense is similar, so I can probably help.

At a minimum you will need two physical interfaces (one will actually do but you will need to know what you are doing!). You need "WAN" and "LAN". OPNSense is still FreeBSD based, I think, so it will not run in a L[inux]XC container for obvious reasons.

Your last paragraph seems rather confused. I don't know what you mean by "glue scripts". VMs communicate via networks

I suggest you try a few experiments to get to grips with virtualisation properly and then move on from there. If you swing by the Proxmox forums with specific issues we'll try to help out but in the end you need to dive in full on ... or not.