Considering you're using Squadrons, I can imagine several unfortunate situations that could've lead to your wipes. But the most glaring of all issues that I can think of would be their AI, particularly speaking: the healer's.
The most common cause of wipe in Pharos first boss is due to the damage taken from crystalline combustion stacks. Squadrons do not do mechanics nor dodge bad stuffs, at all. Their healer AI does not interrupt casts & adjust very poorly to the situations unfolding, i.e. Medica I/II too far back to reach you and/or prioritizing Esuna to cleanse trivial debuff over topping you up from certain death in next 3-5s. They're just that slow. I even had few times when my ACN squadron would Physick (lmao) the tank/me instead because how terrible their AI is.
For Aurum's Coincounter, you can dodge virtually every of their attacks besides auto attacks and their occasional frontal cleave (non-telegraphed). The only way you can get pounded hard here as a tank is if you eat up the large circle AoE. It's guaranteed to take away almost all of your health even as a tank and possibly one shotting your squishier squadron members. My best guess is the healer AI simply do not react accordingly and left you to die, or you just happened to take one of those telegraphed AoE. Another possibility is your other squadron members eats up their lightning AoE and your healer AI decides to neglect you by trying to Esuna all those Electrocution & Paralysis off your squadrons.
Under normal circumstances, as squishy as PLD can be in ARR content, they should -not- have too much trouble so long as cooldowns are rotated well and healers are not sleeping.
Also wanting to add: GNB also have an edge over other tanks for ARR contents for having way more self preservation means. Discounting invulns, they have Rampart, Camouflage, Nebula, Aurora(!!), and on top of that like you've said, there's a heal+shield baked into their 1-2-3. PLD truly feels more dependent at those levels to their healer.