I'm just now diving into the FFG Star Wars RPG. Playing every system that Star Wars has been part of and while each has pros and cons, except the original WOTC D20 which in my personal opinion was a train wreck.

After the Core book what supplement should I be looking at next as a GM?

Mainly looking bang for the buck spent on it.

The consensus will be to ask you what type of campaign material you're looking for, what the basic premise will be, will you be player or GM, etc.

Then, we can help guide you to what will be most immediately beneficial, while ultimately saying, "Buy 'em all!"

The GM Kit would be the first obvious one, that screen can be quite helpful. Beyond that it'll vary a bit on which themes you want to go with all said and done. For Edge of the Empire I'd really recommend Lords of Nal Hutta for a GM. It has a lot of good information about a pretty iconic group of antagonists (Hutts) for a bunch of underworld types.

Career books would vary based on your players, but some, like Lead by Example and Far Horizons contain mechanics in and of themselves. Mass Combat (also found in Onslaught at Arda I IIRC) and homesteads in the previous two books.

The consensus will be to ask you what type of campaign material you're looking for, what the basic premise will be, will you be player or GM, etc.

Then, we can help guide you to what will be most immediately beneficial, while ultimately saying, "Buy 'em all!"

The bank account would say buying them all is a bad idea for now. I'm looking at a GM's perspective for myself. I'm still in the campaign building process.

The timeline I'm targeting is a few years before the Galactic Civil war, around the time the Rebellion is starting to form.

Force will be limited access to the players as its not a good time for force users to be grouped together.

GM Kit would be the next purchase after the core book, but if you haven't played the game yet, I'd strongly urge you to consider the beginner game. It's not just useful for people who've never played an RPG, I've been playing since the original D&D and it was still extremely useful, especially for my players...an older crowd used to binary results and unsure what to do with narrative results. It's a great intro to the rules, has a much longer PDF followup, and comes with a set of dice you will need anyway.

After that ... if you play a lot of social encounters (or want to start doing so), Far Horizons is a really good resource. Otherwise it really depends on the tone of your campaign. They're all good purchases (including those for the other game lines), not a dud yet.

The consensus will be to ask you what type of campaign material you're looking for, what the basic premise will be, will you be player or GM, etc.

Then, we can help guide you to what will be most immediately beneficial, while ultimately saying, "Buy 'em all!"

The bank account would say buying them all is a bad idea for now. I'm looking at a GM's perspective for myself. I'm still in the campaign building process.

The timeline I'm targeting is a few years before the Galactic Civil war, around the time the Rebellion is starting to form.

Force will be limited access to the players as its not a good time for force users to be grouped together.

Oh, you don't need to get them all NOW. I've been at it since the first of the year, and I'm down to just needing 3 books and 1 GM Kit. (I'd actually be done, but for supply issues at the store I've switched my purchases to. Long-ish story.)

With that basic information, I'd say go for the AoR CRB and then look at the EotE and AoR region books (Strongholds of Resistance, Lords of Nal Hutta, and Suns of Fortune).

After that, career books based upon what careers your players are choosing. There's the idea of having players get their own career books, but as GM, I like having access to that information, myself, when the players aren't around.

And dice. Several sets of dice. Or, pick up the Beginner Games, which will have dice and other accessories you might find useful, like maps and character tokens.

The website BeggingforXP.com has some great handouts. In particular, they have a post & handout on what they call "session zero," which is a brainstorming session for a new campaign, aimed at drumming up some of the setting details - with player input - and incorporating that into the character creation process. The idea is that the PCs will be a lot more invested when they've helped create the setting, they'll have a better idea of what character careers & specializations will fit within that framework, and they'll have more intimate knowledge, i.e. no Knowledge rolls to say "does my character know anything?" to kill suspension of disbelief.

To tie this back into your original question, if you can influence the setting location, you might be able to find a sourcebook or splatbook to help you out. For example, if you can get them to buy into the Corellian sector, then Suns of Fortune has a ton of information and plug-and-play encounters to help you out. Said sourcebooks are probably the best "bang-for-your-buck," though the Adversary Decks are really good as well.