Is there a main story like in FFXIV?
Technically, that would be the one for the nation you start in, I guess.
FFXI makes a distinction between quests and missions, where missions tend to be sequential and associated with a starting city or expansion; quests tend to be more scattered, but a few tie into the stories from the missions, and some even require specific mission progress.
The game started with 3 mission lines, one for each starting nation, with a few common sections between them, and used to end in the same fight.
When the 1st expansion, Rise of the Zilart was released, it brought its own new mission line, which required completion of the previous nation mission progress, but also expanded the nation mission lines (I think this is what FFXIV's been doing?...).
So, at this point, you could focus on your nation missions, and after working your way through half of it you could get started on the Zilart ones - along the way you need to unlock some transportation options, but that's probably a lot easier nowadays.
Also, nowadays you can change nations, so if you want you can complete a nation's missions, change nation, and do a new set for a different storyline, while retreading some previous steps - at that point, if you've finished a nation's missions, the early ones for the new nation will be incredibly easy and mostly legwork, unless you're using a lower-level job.
The 2nd expansion was Chains of Promathia, whose missions require only entering a specific zone to start, and unlike RotZ, were meant to be started at a relatively lower level - on the other hand, they used to enforce level caps in its specific areas, but now with a high enough level you can steamroll through most by yourself.
RotZ and CoP actually share a common finale when you end both.
The 3rd one, Treasures of Aht Urghan, like CoP, requires only some talking to the right NPC and travelling to its specific zone to start.
Same for the following one, Wings of the Goddess. this one, however, has nation-based quest lines which you must progress through to be able to proceed with the missions - you can do all of them for more story, but you must complete at least one.
All of these you can pretty much solo to the end at a high enough level - I finished WotG solo with an underleveled weapon, so anything older than it should be viable.
there are also the 3 mini expansions: Crystalline Prophecy, Moogle Kupo d'Etat and Shantotto Ascension - these are mostly self-contained, but the later 2 have fun scenes.
After that the Abyssea stuff was released - it's basically bout beating bosses, nothing fancy, but the areas it takes place in can be nice to help you level and gear up.
And now there's the Seekers of Adoulin, which by the end might require some help, as it was deisgned around more recent levelling practices and gear..
