You got some great advice from @Val_Galahadred already but just to give my two cents as well. I have done a bit of the same mistake as you have by focusing too much on farming toons instead of gear but that also means that you have a solid roster to build on now.
Where to focus depends a lot on which game mode you consider to be the most important for you. Nightmare team will be useful in arena, TW and TB but not so much in raids or for ships.
I think it sounds like a good idea to “finish” resistance (especially bb8) and NS first as you already have them at a very good level and they are both very useful factions. I wouldnt bother with farming Dokuu or with giving DN a zeta though, those resources can be put to better use elsewhere. Palpa unique is nice but not essential, Sion’s is a must have though.
You are an endgame player so I know I’m kicking in open doors now but First Order is one of the most versatile factions you can invest in: they have four good ships (especially silencer but the others are solid too), they are useful in TW, TB, raids (p2 and 4 of sith raid for example) and do ok in Arena. Unfortunately the toons you already have invested in are not the best of the FO toons available but if nothing else you can still farm KRUM for his ship and have them as a side project until you are finished with the other teams (FOE will be necessary and FOO as well).
Good luck!