We have seen Talaxians beyond the Nekrit Expanse. FAR beyond. See the 7th season episode "HOMESTEAD". The Borg could have assimilated a scouting party.
I agree about the Vidiians, particularly because of their medical technology. But their disease has ravaged them for longer than the Borg have even existed. That might make them useless as an assimilation source.
According to Guinan, the Borg are thousands of centuries old. The Phage seems only slightly over twenty (i.e. 2 millennia). Yet the Borg only had assimilated 'a handful of systems' 900 years before the time of Voyager.
Their tech might be very worthwhile to the Borg. Imagine a Borg that can assimilate you from a distance of 40 meters, just by pointing a Vidiian style tricorder at you and beaming nanobots straight into your bloodstream ...