When to start gearing a new team?

With your Phoenix at decent gear, you should be able to get Thrawn next time. He will be a great replacement in your Empire squad.

In my opinion farming Bastila and Jolee would be good. Both are needed to get Revan and are great characters. Both can be used to get Grand Master Yoda to 7* too. Bastila lead, GMY, Ezra, Jolee +1 can be a very useful squad.

Ya leave Phoenix alone, as long as you can get thrawn. They're good but not super useful to power past what thrawn requires. Also depends on what you want these teams for, arena? Cuz arena you'll wanna max out your characters, but acquiring squads for TBs are very useful, as well as factions that have exclusive rewards, like nightsisters and others. Basically figure out what their use is, then analayze how high to power them up

I'd probably push Phoenix to g9 if you can stomach giving them the stun guns. The last tier of Thrawn is certainly doable at g8, but it's pretty tough and you have to clear the stupid waves again every time you fail. A little extra gear would cut way down on frustration factor, and they remain useful as pilots and for special missions in TB.

I like to at least get a squad up to gear 11 before moving on, and take them to 12 if deserved. If you focus your Empire gearing to 11 and then start working on another that you're interested, you can always take Empire to 12 as needed, especially if they'll be your arena team for a while (a lot of them are pilots, so gear 12 is extra helpful). You should try to get R2 with your Empire team (Thrawn helps a lot with this).

Bounty Hunter's are useful in multiple modes, fun, and will unlock Chewie. Nightsisters are fun and useful, too. You'll want a Rebel team, so maybe think about farming Biggs/Wedge and Stormtrooper Han/Leia/Old Ben (these three, along with Farmboy Luke and R2, can get you Commander Luke Skywalker). You have a lot of options, but like others have said, do what's the most enjoyable for you. I like to play fun squads but there's some that just have a lot of utility and will open more doors for you.