How can Venat be the previous Azem?

[Theozilla]

That makes more sense. Was Azem kicked out, or did they quit?

And I didnt realize the second round of sacrifices went ahead too. So it was a third round of sacrifices that Venat opposed?

Azem left because they disagreed with the Zodiark summoning plan in general, but they also didn't join Venat's group either.

And as was explained in ShadowBringers, there was a plan to "cultivate" future life to sacrifice to bring back the Ancients who had sacrificed themselves in the first two rounds of sacrifice to Zodiark.

Here's what Hythlodaeus' shade explained to us in ShadowBringers

The Convocation of Fourteen─well, it was Thirteen at the time─endeavored to create a will for our star. They would repair the fundamental laws of order and halt the spread of destruction.

But creation on such a scale required an immense source of power...

Of those of us who still lived, nearly half offered up their lives in the name of salvation.

And from their sacrifice, Zodiark was born. Just as we had hoped, He reached forth and halted the march of oblivion.

...Yet oh how the star had suffered. So many species lost. The land was blighted, the waters poisoned, and even the wind had ceased to blow.

Once more did our people give of themselves to Zodiark. Another half of our race sacrificed to cleanse the world; to ensure that trees and grasses and myriad tiny lives would sprout and grow and flourish.

The cycle of life had begun anew, and we reconsidered the means by which we might protect it.

The Convocation decided thus: we would nurture our world until it was bursting with vitality. Then, when the time was right, we would offer some portion of its living energy to Zodiark...

In return, He would restore to us those brethren whose souls had fed His strength, and together we would resume our role as stewards.

There were, however, those who disagreed with this plan. They argued that enough had been sacrificed to Zodiark─that this new world should belong to the lives newly born.

These dissidents surrendered their life energies in the creation of Hydaelyn, an incarnation of their opposing belief. And for the first time in history, our people stood divided... Know you, then, how this conflict ended?

Then in Endwalker we also found out that Venat/Hydaelyn also had an additional motivation for the sundering, which was was to also thin the aetheric density of the denizens of Etheirys so they could more readily interact with Dynamis.

Yes. The first one summoned Zodiark and stopped the final days, the 2nd revitalized the planet, the third should have been reviving the dead using the new life...? The cutscenes we got contradict there, the one with venat during our travel back to the present made no sence... as that sacrifices wasnt due until years, or even centuries later as they'd have to wait for the new lifes to grow. And we were told they intended to sacrifice "a part of the new life to restore the lost ones" - theres a line between genocide and taking a part.

Azem left as she opposed Zodiark, we dont know if she was officialy fired, just not yet replaced or if they literally went "f this we dont need her!" and went ahead with their plans...

You have to remember that the cutscene with Venat/Hydaelyn that plays after the Elpis zone is completed is metaphorical and not a literal representation of the chronological events.
As evidenced by the fact that the Final Days was still ongoing (as the sky is depicted in the cutscene) despite Zodiark having already been summoned. It was a condensed depiction of all the major events, not a literal play-by-play. And then last scene with Venat forging on and getting covered in shadow/darkness was an even further abstraction/metaphor of how the later 7 rejoinings weakened her even further.