Re: Phoenix, they seem really overpowered early on, but as you progress through the game they become less of a hassle to fight. Personally, I've found it best to kill Chopper first - it keeps the rest from regenerating protection every time you hit them, so they become much easier to kill.
As others have said, you can skip them, but you'll probably suffer in arena standing and you won't be able to unlock Thrawn (one of the best characters in the game) without them. I don't know the characters at all and had no particular desire to farm them, but I built them up just because Thrawn is that much of a beast.
Unfortunately if you want to progress quickly you'll need to read up on which characters are powerful, or are required to get powerful characters, rather than just going after the ones you loved in the movies. Some of the best characters are guys I'd never heard of until I played this. Case in point: don't waste any more resources on Mace Windu. I love me some Samuel L, but he's absolute trash in this game.
Re: Mods, yeah, they are confusing as hell. I've been at it for a while and there are still some nuances I don't understand, but in general you want as much speed as you can get. More speed = more turns = more win. I noticed you mention you have no mods with speed as a primary stat; each slot has a specific pool of stats it can roll as a primary, and only one can have speed:
Top left (Square): Always Offense
Middle left (Diamond): Always Defense
Bottom left (Circle): Health or Protection
Top right (Arrow): Offense, Defense, Speed, Critical Avoidance, Health, Accuracy, Protection
Middle right (Triangle): Offense, Defense, Critical Chance, Critical Damage, Health, Protection
Bottom right (Cross): Offense, Defense, Health, Protection, Potency, Tenacity
I bolded the stats that are generally considered most useful for the right-side slots. Those aren't hard-and-fast rules - Scavenger Rey has no need for a Potency cross, for example, as she doesn't inflict any resistable debuffs - but by and large, those are the primaries you want to hang on to. Eventually you'll want to only keep 5-dot mods, and some 4-dots with good primaries or speed secondaries. For now though, just try to get as much speed as you can and don't stress about the rest too much. Micromanaging mod stats is a pain and more of a later-game pursuit.
Re: Healers, they become less important as you progress. Later on it's more about crushing the other team before they have a chance to do too much damage. Lumi is handy early on, but she drops off rapidly. I leaned on her a lot back in the day, but she probably isn't worth taking to 7 stars anymore.
Do you have decent ships yet? Most people seem to neglect their ships because they "aren't fun" (I disagree but to each their own), which makes it fairly easy to get into crystal payouts in fleet arena as compared to squad arena. I won't get into a whole thing here about fleet strategy, but I will point out that Unmasked Kylo and his ship are on the same cantina node, and both are extremely worthwhile. Also, you'll need Rebel ships to unlock/upgrade Chimaera, which is something to keep in mind when planning out where to invest your ship credits.
I went to write a quick comment and ended up with a novella, here. Hope some of it helps.
