As far as leveling, roulettes and then level appropriate dungeons are a good way if you have some friends to run them with (in particular if you have a tank friend to run them with) so that you can avoid the DF queues. But the long queue times for DPS are making a lot of people choose Palace of the Dead instead, where you're not disadvantaged by being in the most popular role.

As far as finishing the campaign and moving on from there, if after such a long break you've lost track of quite where you were, there's a new Main Scenario Guide feature to help you find where to pick up on it again.

That will tell you where you need to go once your SAM is at the right level for it. If it's not showing, it can be toggled on/off from:
Character Configuration -> UI Settings -> HUD -> Display Main Scenario Guide

And you might want to go to the "Completed" section of the quest Journal and read through the descriptions of the last few MSQ you'd completed if you want to refresh your memory as to what had been happening in the story. The next few quests will probably make more sense if you do.