Phoenix Squad Help

The general advice is a minimum of gear tier 8, with most of their omegas, most notably their uniques, and leaving out Sabine more because she's hard to get than for reasons of her usefulness.

Minimums are always bad advice; always aim higher. Ezra in particular is a good one to gear above 8, as he's your principal damage dealer, and the most useful Phoenix outside of Phoenix squad.

The Thrawn event is very difficult. Much more difficult than the Palpatine event.

All that said, even if you don't get seven-star Thrawn next time he comes around, five and six star Thrawn are still arena viable and excellent additions to any Empire or Empire/Sith hybrid team. Or, honestly, any team, because Thrawn loves to pop up in other random factions' teams for that sweet, sweet Fracture.