Hi all,
So today support just got back to me.
The Microsoft technical team have apparently looked into the issue and have officially ruled out the link between this issue and the firmware update.
This is very disheartening, considering there are so many of us with the same problem. They have also now told me that I can get a replacement for roughly £600, which again, is insane to ask for.
The support person I was in contact with has been very helpful despite this outcome. They told me that I can pursue an official complaint through the ODR (Online Dispute Resolution): https://www.microsoft.com/en-gb/legal/arbitration/EU-ODR.aspx
I am now making the application to complain and I suggest everyone else do the same.
Furthermore, I think people need to be made aware of this issue through online tech articles or Youtubers. This effectively means that surface devices do not support dual boot options (which is why my secure boot was disabled in the first place). Anyone who tries to download firmware updates while secure boot is disabled WILL have a bricked device.