There is a synergy thread in the crafting section that has been continually kept up to date since synergy released. The first post should have a comprehensive list of every augmentable item and what the possible augments are. Kirin's pole enhancing skill augment is popular at the moment because of 500 skill enhancing skill builds for embrava.
I haven't used a voidcluster personally, but the chatter is that they are quite strong. You could do the 3 cities with 6-8 except maybe tier 4, then a single cluster is probably enough. After that though, you're going to start to need more people. Luckily, shouts continue to be common for Jeuno, at least on slyph.