@jaimbo welcome to Kitz forum by the way.
I used the following as a guide : https://github.com/rcannizzaro/openwrt-netgear-dm200-bridge
The above is for a Netgear dm200 stripped down to bridge mode, also a lantiq chipset, only differences are 64mb ram and a single ethernet port.
In the if you change the profile in the Build.sh from netgear_dm200 to arcadyan_vg3503j it builds and is flashable to the eci
make image FILES=files/ PROFILE=arcadyan_vg3503j PACKAGES="-dsl-vrx200-firmware-xdsl-a -dsl-vrx200-firmware-xdsl-b-patch luci-mod-admin-full luci-app-statistics collectd-mod-uptime luci-proto-ipv6 luci-proto-ppp luci-theme-bootstrap uhttpd"
That's what I'm working with, there's also network files to adjust.
Thanks for the welcome

So I tried as you suggested, used the files in that github repo, slightly modified to change the IP addresses of the network file to match my IP ranges so I could access the UI etc. (Changed "option ipaddr" for "config interface mgmt" to an IP within my range)
Managed to flash using the instructions here:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=17094e9e642ffca19c6b6681766d2bcbb8e6e565 (though the instructions in that post are slightly wrong, the command "erase 0xb0020000 $(filesize)" should actually be "erase 0xb0020000 +$(filesize)", but figured that out and got there in the end)
Could see the OpenWrt boot process through the serial console and it looked okay to me. LAN1 stopped working, LAN2 appeared to be working, but no matter what I tried, I couldn't connect to LUCI and just leaving everything plugged in as it should be, the line never seemed to connect

(Also either the power LED or DSL LED was flashing green at the time)
Anything you can think that I am missing? Back on jsamuels build for the time being, but would like to give a newer builld another try!