What I find helps is to flag one character as a Journey character so all the sub-characters needed are highlighted. Then manually flag five characters for a team you want to work on, plus one character needed for the Journey. Buy and farm any and all gear that is flagged as useful for them until they are at a level you're comfortable with, then unflag and move onto either the next team or the next Journey character.
If multiple characters can use the same piece of gear, I prioritize as follows:
The journey character.
The squad character with the lowest gp. (This way you finish a squad at about the same time)
Any other character needed for the Journey.
Any other character that I like.
You'll notice that with that prioritization I don't horde gear. That's intentional. Part of the reason I think people lack squads is because they're afraid to dip into their horde. I just gear up everything and deal with it later.
A few caveats--
Ideally, the squad you want to work on will be characters that interact well with whatever your Journey is. So if you're going for Padme, you're working on Galactic Republic. But, later, say you're going for Jedi Master Luke. You farm up so many characters to get him that if you're also working on Rebels or Jedi or even Empire, you can compliment a bunch of teams that use the characters needed to get Luke. This isn't much of a big deal with Rey or Kylo since they both mostly use characters of their own faction, and those factions don't mix well with other teams. But, you know, for later!
"A level you're comfortable with" is a pretty iffy statement. That's because it's entirely subjective. If you're working on getting Chewie or Padme, you could probably be safe stopping at g12 before moving onto the next batch of characters and come back to them later when you're ready to farm hellllllla relics.