I'm in a very similar situation myself! I had to take a break from the game due to real life stuff but when I came back, I decided to go ahead and purchase the Heavensward expansion since I was enjoying my time and was nearing the original level 50 cap. I figured I would jump into one of the new classes only to find out they're locked until you actually open up the Heavensward content as you mentioned. So now I'm picking away at the story content still trying to make it to the expansion areas.
I feel like FF14's original 1-50 story had pretty good pacing from what I remember and it's a real shame how much I find myself dragging my feet on completing the newer quest lines. I imagine this issue is exacerbated by the fact that the content in between the 1-50 portion and Heavensward was doled out over a number of patches and playing through it with breaks in between releases was probably much more tolerable.
I will say that whenever I find myself not wanting to go initiate the next line of main quests, it's pretty easy to find something else to pick up whether it be optional side quests, gathering/crafting, the Gold Saucer, etc. Now, whether that stuff is actually enticing to an individual player is another question of course.
Eon_V's comments