Agreed that you're not midgame until you're level 85. Your first 4 months to a year at level 85 (depending on spending and how intensely you play) are probably best described as the midgame. Another way to measure it is to say that somewhere between 1.5M gp and 2.5M gp you enter the midgame (depending on how focussed your roster is) and somewhere between 3.5M and 4.0M you enter the endgame.
As for advice, do not neglect Bounty Hunters, and while you're going to be pressed for priorities, I'd also go Ewoks early.
Why? They're easy to farm, they do very, very well at low gear but lose value compared to other squads at high gear levels. You can put them off if you want, but you're going to do them anyway to get C-3p0 (who is an absolutely crucial character for success in the raids), and if you grab them late, you'll just use them in the C-3p0 event and hardly ever again. If you get them early, you can get a lot of use out of them before the game passes them by. Also the ewok event that gives you Wicket shards will give you Zetas as soon as you finish Wicket.
Bounty Hunters are also crucial for Chewbacca and the Millennium Falcon AND they you cash, they are important for a long time in GAC, and you're going to need Bossk no matter what because he has the best ship in the game.
I've also written other advice on early priorities. But my most important advice? In a thread several months ago I said this:
if you want to because that's more fun to you, then do the fun thing. Always choose the fun thing. It is way too easy for the game to feel like a job with all the long-term work needed to collect and gear characters, so anything you do to make things more exciting and rewarding emotionally/ personally is always going to be more important than the things you can do to shave 7.2% off the farm time of a toon you never really liked in the first place.
