I was going to suggest the Great Attractor or the Shapley Supercluster, but I think your suggestion is better. It’s more point-like and since it’s farther away (well outside of the reachable universe) it results in a more uniform set of directions over long distances.
Of course, cultural influence will be big. If these explorers are Terragen then most likely the Milky Way’s north/south direction will be pretty deeply ingrained in their coordinate systems. They might keep on using that, since it’s not like manual astrolabe-style navigation will ever be relevant at that level of technology.
By the time intergalactic navigation is relevant we’ll have likely dismantled Earth. The vast majority of it is just sitting there generating gravity, a huge waste of its potential.