>>684001
Astral is a combination of two separate effects.
One is the buff you are actually stacking by performing Quick Assists. This buff works as written in the description of the set, and is entire squad-wide, lasts 15 seconds, with all stacks duration being refreshed on Quick Assist.
i.e. if you refresh a three stacks buff by getting another Quick Assist in before the 15 seconds run out, you'll get to keep all three stacks for another 15 seconds at least, otherwise you lose the entire stack and have to start from scratch.
This effect, however, doesn't do anything directly and is only used for calculating the damage of the second effect - which is the actual DMG% buff you get when they Quick Assist in.
The DMG% buff is personal and it's not clearly explained in the set's description, but it also runs for 15 seconds, however its duration is completely independent from the first "stacking" effect.
To give you a more specific example of how it actually works, let's say you have Agent A and Agent B:
>Astral 0 stacks; Agent A: 0% DMG buff; Agent B: 0% DMG buff
>Agent A QAs in ➡ Astral 1 stack, 15 seconds left, Agent A: 8% DMG buff, 15 seconds left; Agent B: 0% DMG buff
>2 seconds pass, Agent B QAs in ➡ Astral 2 stacks, 15 seconds left; Agent A: 16% DMG buff, 13 seconds left; Agent B: 16% DMG buff 15 seconds left
>10 seconds pass, Agent A QAs in ➡ Astral 3 stacks, 15 seconds left; Agent A: 24% DMG buff, 15 seconds left; Agent B: 24% DMG buff, 5 seconds left
>2 seconds pass, Agent B raw swap in ➡ Astral 3 stacks, 13 seconds left; Agent A: 24% DMG buff, 13 seconds left; Agent B: 24% DMG buff, 3 seconds left
>5 seconds pass, Agent A raw swaps in ➡ Astral 3 stacks, 8 seconds left; Agent A: 24% DMG buff, 8 seconds left; Agent B 0% DMG buff
>5 seconds pass, Agent B QAs in ➡ Astral 3 stacks, 15 seconds left; Agent A: 24% DMG buff, 3 seconds left; Agent B: 24% DMG buff, 15 seconds left
>16 seconds pass, Agent A QAs in ➡ Astral 1 stack, 15 seconds left; Agent A: 8% DMG buff, 15 seconds left; Agent B 0% DMG buff
>>684011
>and is practically spamable
Spamming assists is a damage loss, especially in timed content as the Vital View time slowdown doesn't actually slow down the timer.
Assists have to be used purposefully and are rarely optimal when taken at every given opportunity.
Pulchra in particular has her own off-field presence she benefits from not having interrupted.
Additionally, your resources are limited and Evasive assists is not something you can do during the burst, so getting a QA off will require you to spend EX, which is, again, a resource and a Decibel source, so those presses should also consider your broader Ultimate rotation.
It's very easy not to have a good way of prompting QA with a specific agent during the 12s stun rotation, given your only window of opportunity before that is 3 seconds long.
Also, as was said before, if you're running Pulchra and Trigger in the same squad, you aren't looking just at "easier Astral uptime" - you're looking at having or not having the buff on your Pulchra to begin with, assuming your team order is DPS/Pulchra/Trigger.
On top of that, while I do agree that there are times where going for Trigger's QA prompts is suboptimal, I'm highly doubtful there's is currently literally anything more optimal than going for her sniping finisher when specifically opening a fight with her.
Chances are, you'll be starting with sniping anyway because you need her gauge to enable the 35% Stun DMG Multiplier ASAP, and whatever else you'll be able to do during the 1.5 seconds it saves you to have her fire her finisher off-field cannot compete with instantly skipping to the second stack of Astral right then and there.
With the DPS/Pulchra/Trigger sequence in particular, you'll be following up with Pulchra's EX > DPS QA, meaning you'll have the full 24% Astral buff on both your DPS and Pulchra right off the bat, which is virtually guaranteed to be more valuable than the extra 1.5 BA you'd land in the same time frame.