I’ve been investigating and unfortunately this kind of filtering is not currently possible.
There is an unexported related.Document that is the basis of the Related Content interface.
But this is not reflected on the .Page.Type meaning that a .Page which is part of Related Content cannot be differentiated from a .Page that isn’t.
I suppose that my only option to achieve this kind of filtering would be to manually assign a front matter parameter to content files that will not be part of Related Content.
This is totally random spitballing here (on phone with no access to laptop, so preemptive apologies), but how about good-old-fashioned len with $related to check if related pages isn’t empty?