Doubt: Level, Stars, Gear

You don't really need to worry about level. You'll want characters you use at 85 eventually, but beyond that you can pretty much just add levels when it's necessary in order to add gear or ability ranks. Gear is the main thing that will make your characters noticeably stronger.

And just farm characters you use to 7 stars. Just do it. It takes a while but once it's done, it's done forever.