Attempting to access the disc whether it’s the actual disc or a ripped version using Decrypter results in DVD Shrink telling me that it’s an ‘invalid DVD structure’.
So… Clone DVD will read it but it doesn’t have VBR like DVD Shrink, just a CBR. I tried copying it at 100% using Clone DVD then seeing what DVD Shrink would make of it. It will allow me to access it now but can only shrink it to 99%, leaving it still just under 6GB and being unable to go any smaller! Trying to manually compress it as opposed to auto compression does nothing either.