Well I’ve had another go this morning and believe I’ve managed it. My problem seemed to be that I already had 5 devices in Mullvad through my tinkering, so I deleted one and made a new one (just as an fyi in case you hit the same issue).
So I followed this guy on YouTube to set up an Openwrt router VM https://www.youtube.com/watch?v=3mPbrunpjpk&t=897s
When you are able to route traffic through the VM stop following the tutorial and use this link instead to set up the VPN https://mullvad.net/en/help/running-wireguard-router
And apparently I’m now running the Openwrt router through Mullvad.
I did all the SSH parts in Console and I put my public key into the website through the Mullvad link above and copied the IP addresses from the same page.
So theoretically I just have to set vmbr1 as my bridge to containers and VMS that I want to run through my VPN and set up port forwarding for them in the OpenWRT interface and they’ll route through Mullvad
Hope this helps.
Have a look at my reply to the other guy, I’m pretty sure I’ve got it working now