Help with Routing via Proxmox Linux Bridge to Opnsense VM

Hi all - I have Proxmox 8.3 running on a dedicated server with a single Gigabit connection from the ISP to the physical server. VMBR0 currently has the public IP configured on it, so I can reach Proxmox GUI from the browser.

I have created VMBR100 for my LAN interface on the Opnsense (and for VM LAN interfaces to connect into). I can ping and log onto the Opnsense GUI from another VM via LAN interface no problem. However, when I move my public IP onto my Opnsense node and remove it from VMBR0 - I lose all connectivity.

I have configured NAT, ACL and default routing on the Opnsense appliance to reach my VM's and Proxmox server via HTTPS and SSH but I never see ARP resolving for the default gateway of the ISP on the Opnsense.

I even configured the MAC address from VMBR0 onto the WAN interface on the Opnsense in case the ISP had cached the ARP for my public IP (this trick used to work when customers migrated to new hardware in the data centres, we would clear the ARP table for their VLAN or advise them to re-use the same MAC so the ARP table does not break).

Here is my /etc/network/interfaces file and how it looks when I removed the public IP, is there something wrong with this config?

auto lo
iface lo inet loopback
iface eth0 inet manual

auto vmbr0
iface vmbr0 inet manual
        bridge-ports eth0
        bridge-stp off
        bridge-fd 0
        hwaddress A0:42:3F:3F:93:68
#WAN

auto vmbr100
iface vmbr100 inet static
        address 172.16.100.2/24
        gateway 172.16.100.1
        bridge-ports none
        bridge-stp off
        bridge-fd 0
#LAN