OP, I know you've had a lot of vitriol hurled at you and I get that this has all been a disheartening experience. After all, I don't know you or what you're going through in your life. I would like to believe that you are sincere and genuine about your struggles, even if much of it perplexes me.

But most of all I don't understand your hangup about having to maximise uptime in a story dungeon - including on runs where you're using NPCs. Who exactly is here to judge and berate you for not optimally squeezing in one last Phlegma with microseconds to spare before an AoE puddle explodes beneath your feet? Who cares if your Phlegma overcaps by like a few seconds? The NPCs will obviously not care, and no reasonable Duty Finder group will be parsing you or get on your back about it unless you're literally just standing there semi-AFK. You're so hung up on your perceived shortcomings based on past harder Endwalker content that you've needlessly brought this mentality over to a story dungeon to the complete detriment of your ability to focus on a boss's telegraphs and to learn how to adapt and respond to what is essentially a recycled mechanic. Walk before you can run. Get familiar with the basics of the fight first, THEN think about optimisation.

You talk about focusing on your SGE "rotation" when all that amounts to is keeping your Eukrasian Dosis DoT up, spam Dosis III and pop buttons like Phlegma and Psyche when they're available, with Toxikon for mobility. You can afford to take your eyes off your hotbars for a moment and just mash Dosis III while you watch out for the boss is about to do next. Your GCD shield casts are not part of your "rotation"; they're things you proactively throw out in anticipation of an incoming mechanic, and even then your first resort should be your oGCDs like Physis and Kerachole. You needlessly spent a second popping Eukrasia in one instance while trying to slowly amble your way out of danger when you could have thrown a quick Taurochole on yourself instead. It may be worth a review of the fundamentals of SGE and your setup, because there's no reason to devote so much time to staring at your hotbars as though you're a complete novice to the job.

You've had a lot of tips about how to handle this particular boss fight, so I don't presently feel any need to regurgitate a lot of what is already said. No matter how you approach it next (get a premade? Take a short break first to clear your mind?) I wish you the best of luck for it and for the future encounters to come.