Nothing will happen in a month that is not already happening so delaying is not really winning. New empire characters seem ok but not gamechangers, then there is the new character that will come with ability never seen before but we don't know when and does not mean it will be op toon. For god's sake even on decent shards that are coming of age they still run wiggs and make it to top 10.
So zaul is still in good shape and will be for a while. People that invested in sith 2 mos ago got bigger return but still not to late imo. In terms of zetas, it is zaul for sure (lol, ha ha) but others use with caution. DN's unique is definitively zeta you don't want to spend. Benefit is really really not worth it. Zavage has a large fan community and is probably decent but i personally prefer Zylo in sith team for a number of reasons. Zid is fun and lights up the game with red light special but not essential. DN is cream of the crop and he will live long after sith meta is dead, so getting him is a must. ST is ok, but not really essential if you have zylo or zavage. I did not talk about Dooku as two of us really dislike each other but he is actually not all that bad and ai can't mess up much his play so generally thumbs up.
Other option is rex team for which u don't need zeta so more afordable option. Re GK is it hard to say, he can be useful but most people in top 10 don't run GK any more even if they have them. GK is also way better when you play against them than when u actually play him yourself. If I was on tight budget would pass on gearing him up for top 10 arena. But if you are shooting for top 50 yes he can definitively make a difference.
Long live the sith and something tells me that even for christmas you will be able to run sith and place well (like top 20) in arena against new first order and resistance teams.