First, you can use Sion at 3*. He works fine without Nihilus. Test him over TFP for a little while. If you go up leave him in, if you fall take him out until he is farmed. Sion does need his zeta though.
My my rank 60 Nightmare team got torched by your exact team today though, so it isnt bad. Get your Vaders speed up. Use the mod sort tool on .gg and put your fastest mods on Vader. I see +14 Speeds on Farmboy, Clone Chewie and Wedge. Steal those. R2s +16 also if you are not using him in Arena. Get Vader up to 220+ speed and you should rise even with your current team.
I know its kind of a chore, but before every TW cycle I go through my mods and move them around. Your teams and your guild will thank you for it. We had a guy running a 100k Nightmare team that lost first time every TW. He had wretched mods. They matter. A lot.