I farmed JKA (and JC) to 7* mainly from hard nodes, because Cantina energy is too valuable. JC is easily farmed in hard nodes, and JKA just is too expensive at a 16 energy node. Meanwhile I have GS, Lando, Wedge, TFP, Finn, Kylo at 7* and B2 and Aayla at 4* from Cantina. Farming JKA in Cantina simply takes away too much.
I am doing RG from hard nodes now (got a lot from Bronzium, Cantina energy is too expensive here too), and then going for Dooku and the two first order hard node toons. In Cantina I am finishing Aayla and then going for Dooku and B2.
Other interesting toons for special purposes in Cantina are Jawa, Old Ben, Scarif and Chirpa, but I bet there will be interesting additions in the next months as some very strong toons will become f2p sooner or later (Chirrut, Baze, Deathtrooper, Nihilus etc. are must haves).
For Zeta I did Maul first, and will probably do Nihilus unique next in a few days. Vader would be better for raids, so I might doing him before Nihilus, but only because I'm pretty safe on Rank 1 every day. The rule for farming, gearing etc always is Arena first.