Again, if the boss isn't about to cleave, it's generally recoverable without too much issue. Of course this depends on your offtank being awake
This is precisely why I feel that recording your own play is so important. It can be hard to see the big picture and get a feel for the flow and timing of the encounter whilst you are in the thick of it trying to learn with your eyes on stilts. Watching your own replays back not only allows you to spot your own mistakes, but also lets you revisit the encounter with a calm and clear head which makes it much easier to learn and build a stronger plan.
A9S has a decent number of occasions where you'll get little to no tank damage for as much as 15 seconds. They aren't quite as extreme or obvious as A10S or A12S, but they are certainly there. From what I remember back 100 pages or so was that a larger part of your issues in A9S stemmed from needlessly casting succor?