So the PC connected to opnsense is running proxmox for it's OS?
Create a bridge for each physical interface, then add a tagged interface to it for the one connected to opnsense;
Eg, vmbr2 could have enp2s0.100 and enp9s1f0 as members. Just add .vlanid to the end of the interface name in the bridge settings in proxmox, and don't make the bridges vlan aware.
If vmbr0 is vlan aware then just add vmbr0.100 instead of enp2s0.100
With that setup the server will switch packets between the vlans on enp2s0 and the other interfaces. Don't need to put any VMs on the bridges
Will add: this is using the PC like a switch, you're probably better off using an actual switch with vlan configuration instead
So the PC connected to opnsense is running proxmox for it's OS? Create a bridge for each physical interface, then add a tagged interface to it for the one connected to opnsense; Eg, vmbr2 could have enp2s0.100 and enp9s1f0 as members. Just add .vlanid to the end of the interface name in the bridge settings in proxmox, and don't make the bridges vlan aware. If vmbr0 is vlan aware then just add vmbr0.100 instead of enp2s0.100 With that setup the server will switch packets between the vlans on enp2s0 and the other interfaces. Don't need to put any VMs on the bridges
Will add: this is using the PC like a switch, you're probably better off using an actual switch with vlan configuration instead