The Zeta on Jyn's unique is simply that she's immune to stun, and seeing as how the whole point is to keep the Rancor from taking any turns at all, you do not need her zeta. If you do get very unlucky and she gets stunned, the rest of your team should be able to keep the Rancor's TM reduced enough until she gets out of stun, which won't take long. She just needs to have high crit chance and high speed.
I'm not quite at a full solo yet, but from personal experience and from what everyone says, Jyn is by far the most valuable for soloing Rancor, so you'll be in good shape once she's at 7*. Even with no other TM reducing characters I can get through the first two phases with just her, Wiggs, Cassian and Ackbar, all at gear 8.
Also, I would definitely use Tarkin in place of EP or Sidious. The guaranteed 50% TM removal from his Ultimate Firepower is excellent as another "oh shit" button. Having offense down on the Rancor is also invaluable since like someone else said, Jyn dying is one of the biggest issues, so cutting its damage in half is very useful for when it manages to take a turn.
Final team I'd use is Zader, Jyn, Rex, TFP and Tarkin. That's pretty much what I'm heading towards if I decide to put my first zeta on Vader.