[Spoilers inclluding 7.0] My "True" Main Character Theory

[MistakeNot]

Yes, I would count it separate. If we are to include it it would be too early to say who can possibly be the main protagonist for 6.x as we haven't seen the entire plot.
It may as well be Zero, or it can even be Golbez. To me the plot in each post-credit is quite disconnected from the pre-credit.

Oh, no.
Previous expansions each story started before the expansion actually came out, and ended half-way through the post-credit patches.
The Heavensward story started in patch 2.4 (when we start looking for Lady Iceheart) and ended in patch 3.3 (Final Steps of Faith)
Stormblood story started no later than patch 3.5 (meeting the Griffin, and Baelsar's Wall), and ended in patch 4.3 (Castrum Fluminis)

Endwalker is a bit different in that the main story (and the whole Ascian/Zodiark overarching plot) ended with patch 6.0, with patch 6.1 and onwards (probably) serving as setup for 7.0

[Cleretic]

Oh, no.
Previous expansions each story started before the expansion actually came out, and ended half-way through the post-credit patches.
The Heavensward story started in patch 2.4 (when we start looking for Lady Iceheart) and ended in patch 3.3 (Final Steps of Faith)
Stormblood story started no later than patch 3.5 (meeting the Griffin, and Baelsar's Wall), and ended in patch 4.3 (Castrum Fluminis)

Endwalker is a bit different in that the main story (and the whole Ascian/Zodiark overarching plot) ended with patch 6.0, with patch 6.1 and onwards (probably) serving as setup for 7.0

Truth: it genuinely took until someone told me sometime when I was playing Stormblood that Heavensward ended at Thordan, not Nidhogg.

Admittedly I was basically alone as far as any community goes until about the time I finished 5.0; I had a couple of disconnected friends but was mostly going solo. (I remember it was 5.0 because I had a very weird, rude awakening when bringing out my general take of 'finished Shadowbringers, shame the ending dropped the ball because I loved all the rest'.) I admit I'm far from the usual example, but it means I can attest: if you don't have some form of external or metatextual confirmation that this is where 'the expansion' ends, it's genuinely hard to tell. Especially if you're like me and both skip the credits and swiftly forget that you did.