The switch is quite capable and supports something extra using the dsa driver. I won’t take this into account for this. It would enable to have every ethernet port as a real eth device in linux.
The rest is based on the OpenWRT swconfig style configuration.
The SoC has 3 ethernet devices. eth0 to eth2.
I assume eth0 goes to switch port 5, eth1 to port 6 and eth2 is WAN.
The switch has 7 ports. 2 are connected to the SoC. 5 are available as LAN1 (port0) to LAN5 (port4).
The third ethernet port is connected to the WAN interface. It is switchable between copper and SFP.
config 'switch'
option 'vlan0' '0 1 2 3 5'
option 'vlan1' '4 6'
Then you have eth0 as LAN and eth1 as WAN.
If you add eth0 and eth2 to you lan definition in OpenWRT you got what you wanted.
No VLAN tags in software involved.
The bridge will for most users already be there for the Wifi interfaces so no new bridge.
Edit: This config may even be better. The SFP port should be able to support 2.5GE so adding a SFP module would possibly allow connecting a 2.5GE switch or device like your shiny new server or something like that.