Ok thanks.
The internet box 2 also has an SFP cage but supports g.fast over vdsl directly without using the SFP unit. I just happened to be sent one through the mistake of Swisscom.
I am assuming that getting the device recognised, firmware injected etc. by the Edgerouter would be enough to have it act like a standard SFP and the g.fast side of things is handled transparently. Does that sound reasonable?