Can't find where to unlock them I even researched power of the endless thinking it'd unlock the last tiers of ships.

It's in the same research 'quarter' as your other ship hulls, various tech for expanding empire. It's the left quarter and the dreadnought is the second ring from outside I think. The other, but still large, ships come slightly sooner. You can also research tech to add additional modules onto your existing ships, which is a nice touch.