Everyone here is going to give you good advice, but what we can't know is what will make the game fun for you.
For instance, you have some farming of shards to do to collect SLKR, but even though you're technically closer to SEE, the large number of relic mats you need to gain means the farming for SLKR toons won't slow you down. On top of that, the gains from SLKR (which include soloing the Heroic Sith raid) outpace the gains for finishing first in Arena every day.
So from a pure power efficiency perspective, SLKR should be the first GL for everyone.
But the point of playing a game is to have fun. Some people don't like the new movies (in which case maybe both SEE and SLKR are out) or maybe they just like playing LS heroes more than DS (in which case JKL is a great toon to pursue), etc.
So really, the first question for you is, what is your goal?
If your goal is finishing first in squad Arena, then SEE or Jedi Master Kenobi should be your very first goal.
If it's general roster improvement, then Fleet Arena improvement followed quickly by SLKR to Rey (Jedi Training) to Jedi Knight Luke might be your goal.
If it's preparing for Dark Side Territory Battles (which is actually a good goal for you, since you should want to get into a good guild, and for where you are they should be doing DS Geo TB) Then Grievous, Geos (esp Brood Alpha, but all of them), Nute, Dooku, Ventress + Separatist ships on the side would be the place to start.
I'm not trying to confuse you, just saying that a lot of people are going to have a lot of good ideas, but they won't necessarily be the most fun for YOU.
So tell us what you most like about the game, the game modes you most enjoy, the characters you most enjoy, and then the general good advice can become quickly focussed on advice that works best for you personally.
Perhaps the easiest way to get some focus is this: when you decided to come back, what was it that made you think, "Yeah, let's start this account back up again." Whatever comes into your head first, just tell us, even if it sounds silly. (Some people do enjoy taking low power characters to absurd levels, like getting all the Tuskens to relic 7. If that's what makes you happy, just let us know & we'll help you get there.)