I have finally forced myself to edit the video, so I made a cut of this run as well.
For running Astra + S11 together, specifically in Deadly, I think I have finally found the mostly optimal way of using Astra's Ultimate, at least for S11 with The Brimstone.
With Nocturne (which I don't have), I'm assuming you just want to spam all the CAs back-to-back, ideally with EX presses and maybe BA combos in-between, but with The Brimstone I was always running into the issue where trying to fit in Astra's CA would require you to cut down on The Brimstone stacking (as there's simply not enough time for that, even with Lighter's stun extension), and doing so would bring the overall damage output down to the point, where I'm not sure you're even getting anything out of them.
The solution was simple and I stumbled on it mostly by accident - just don't.
Don't do Astra's post Ultimate CAs into the stun.
Dump S11's Ultimate first and then use the CAs she gets from Astra to more aggressively build Decibels for the next stun.
Against Pompey, I would routinely get four stuns but only enough Decibels for three Ultimates. And to do even that, I would need to do dump my EX and trigger Burns, on top of getting at least one Defensive Assist.
And by the time of the fourth stun, I simply would not have enough resources to build an extra Ultimate, as the fourth stun usually has to happen rather quickly after the third one, leaving too little time.
But with the CAs from Astra's Ultimate you get so much extra Decibels, preparing an Ultimate in time becomes way easier.
And because you aren't in a rush to fit them all into the tiny stun window, you can actually stagger them up properly to keep The Brimstone stacked.
The run wasn't even optimized fully and I got 3k score increase over my previous best.
The obvious difference is 4 stuns + 3 Ultimates vs 4 stuns + 4 Ultimates, but the 4-Ultimates run actually had a higher total score even before the final Ultimate has landed.
So it's not just about the Decibel buildup - it's a strictly higher DPS rotation.
This is a bit Pompey-specific, but I think the general idea would apply to other runs as well. And possibly Shiyu too, provided you can't kill in a single stun.