Time is Running Out for the Ascian's Plan?

[Iscah]

I still don't know where it is at that the Ascians are like, "We need to hurry hurry hurry, y'all!" Can someone point to the point in the story where it happens?

I also don't think we've been told this - unless someone has direct quotes, I'm inclined to chalk it up to the same ongoing misunderstanding that some people think the Eighth Calamity is enough to release Zodiark when that was merely fan speculation on the meaning of a pre-Shadowbringers promotional interview that said we needed to prevent the "eighth and last" Umbral Calamity. (More likely interpretation now: it would be considered the last in the other timeline we prevented - either because the timeline allegedly ceased to be, or because it stuffed the world up that badly that there won't be anything left to bring about a ninth.)

There was urgency to their plan to rejoin the First, at least at the point when Elidibus started manipulating Ardbert's group, because that came so very close to a flood state and they couldn't let that happen. But beyond that... well, unless it was all bravado, Emet-Selch specifically tells us that even our interference in the First is only a minor setback to their plans. They have the patience to wait for millennia for their plans to reach fruition, even if they'd much rather we stop making a mess of them now.

I'm still also inclined to take anything they say with a grain of salt, because like, Lahabrea was all, "We will need the primals in order to get the calamity going." But like, nah, not really. They resort to a chemical weapon to kick off the 8th, which did happen in one of the timelines. No primal involved at all. Ascian words are as air.

The two methods aren't as different as you're simplifying them to be.

The Ascians' ultimate aim is destabilising the world's aether to the point that it triggers a calamity. Primals - and stoking people's fears to encourage them to summon primals - are means rather than an end goal.

Meanwhile, Black Rose is very much an aetherial weapon, not just a chemical one. It's not a poison - it kills by halting a person's life energies so they stop breathing. It shifts energies towards umbral/Light, making it a match for the First's Light-soaked state. That what made it seem ideal for Emet-Selch's plan - although in the end it sounds like it may have worked too well.