OPNsense to "bridged" modem over WiFi

[stoneforger]

Hello,
this is maybe not particular to OpnSense but if anyone could chip in that'd be much appreciated.

I have an OPNSense VM running in a TrueNAS installation, Intel card and two emulated Intel interfaces on that PHY card on the VM ( one is the LAN the other is the WAN if ). There's another Realtek interface that's used only by other TrueNAS VMs/jails.
Currently I have a Deco M4 pair to push traffic between the OpnSense ( mainly used as DHCP ) and a cable modem in bridge mode - the DECO M4 does the actual routing and is the gateway to the internet.
My question is if I set the WAN interface on the OpnSense in a new VLAN interface, would that possibly allow me to talk directly to the bridged modem so that the WAN interface MAC address is what the modem sees and then I can use the OpnSense as the router/fw and just set the DECO as a WiFi AP ? Is there some other way to do this? I also have some Ethernet over the Powersocket devices which I could maybe use to wire directly the OpnSense to the modem. The reason for preferring this is that the WiFi seems to be substantially faster than the Wall Socket ethernet devices.
A rough schematic of my "wiring" is attached. Thanks for any help/pointers.