I'm level 50 and my light side squad is Chewie, Ahsoka, Luminara, Jedi Consular and Clone Trooper.
You don't need any new heroes, just use the ones you've got.
* except Luminara, she's the bomb.
I'm up to 5F currently and I've been 3 starring missions first try because it's so easy. If I wanted to I could probably finish off all of 5 and 5 hard tonight.
You're doing fine, just level, star and gear your top 5 and you'll be fine.
You only need to unlock new levels for gear, so don't waste your energy trying and failing when you could just wait a few days the 3* it.
Also, it will be a long wait until they make any new levels, so stretch out the enjoyment for as long as you can or you'll just be simming then for weeks without any new content.