Squad/team composition

I'd keep farming phoenix to upgrade your Thrawn to 7*. Thrawn is a very strong character, who fits well in most teams.

For arena I'd suggest to either switch back to jedi (zBastilla lead, GMY, Ezra, Old Ben and Thrawn fir a start) or develop your empire team. zEP, Vader, Tarkin, Thrawn and TFP can still take you to rank 200. (But your jedi will probably be stronger).

No matter which team you decide on using (and even if you keep your current Finn lead team) you shold distribute your mods better. Speed is the single most important stat in arena. Having key characters like Poe, R2D2, GM Yoda or Thrawn move first is what can win the battle for you - or loose it, if your opponent moves first. Leaving all those speed arrows on random characters instead of on your arena team is a huge mistake:

https://swgoh.gg/p/126257955/mods/?stat1_5=1

https://swgoh.gg/p/126257955/mods/?stat2_5=1

Switching to a new team without sorting your mods is a wasted effort. Mods make a huge difference.

Does auto assigning mods work? Or do I need to read up and do it properly

You should read up and do it properly if you want good results.

I’m getting there! I’ve read up on set bonuses, what the A-E ratings etc are so I’ve redistributed what I’ve got. Need to read up on slicing etc now.

Trying to focus on speed mods if I can.

Should you try to have a mix of mods/set bonuses across your team members?

Mod set types and stats should reflect each individual toon's role and abilities and the way you use them.

Tank? Health, protection & defense, even at the expense of speed in many cases

Attacker? Offense/Crit Chance/Crit Damage mix depending on their abilities & your preferences / available mods

Support/Healer? Health & protection plus potency if they need to land debuffs

If the character has debuff abilities you rely on, give them a potency set &/or cross.