With what you currently have — zEP lead with BSF (get her starred up), Vader, Thrawn and Dooku/Tarkin.
BSF supercharges EP especially once she’s Zeta’d and 7*. The other advantage here is that she will also fit with Darth Revan in the future....
The idea with EP, Bastilla Fallen and Dooku is to spread shock everywhere and tons of debuffs for Vader to Cull. Thrawn can Control with Fracture and provide protection recovery.
—> Longer term the easiest team to build for Arena is probably Padme + GRepublic. You need Separatists to unlock her. My suggestion would be Nute lead, with B2 + Sun Fac, Spy and Soldier. (This gives the 3 Geo pilots for Fleet purposes...farm their ships!). Longer term you will want a full Geo Squad as well as Separatist droids.
Personally I used Nute (lead), B2 (Control/dispel) Sun Fac (Tank), Spy (Damage), and Ventress (Damage, Dispel, Stun)
Going with Padme - you need JKA and GK (HAAT raid) + 2 other GRepublic. Most commonly Ahsoka and one out of C3PO/GMY/R2
—> Padme does need her Zetas to work well. But in terms of “countering metas“ her team and a Relic’d GG Nuke team are the best options currently.
