When should I drop tank stance?

[Lynesse]

Ideally, you switch after a single aggro combo. This assumes you have adequate gear relative to your party and they're considerate enough to being utilizing their own aggro tools. I recommend doing the standard one aggro combo/switch stances and see how long it takes before people start catching up. If it's fairly quick, you'll need to perform two aggro combos before dropping stance.

This, pretty much.

For bosses, I start in Shield Oath, then Shield Lob -> Circle of Scorn -> Flash -> Aggro combo 2x -> Sword Oath + Rampart -> Goring Blade combo under Fight or Flight -> Royal Authority combo 2x.

This is what I try to do. Sometimes I only use one aggro combo. Sometimes I fumble things and start the Goring Blade combo too early. Oops.

I find that people almost never use their aggro tools. This keeps me ahead, and it is noticeable how much damage I do in Sword Oath. I do occasionally replace Royal Authority combos with Rage of Halone combos if I feel uneasy.

I also try to preempt add phases by switching back to Shield Oath before the adds spawn -- but I have successfully grabbed and held aggro in Sword Oath. I've also been unsuccessful, which is why I started trying to preempt things.

What Hierro said about cooldown usage is absolutely, 100% right.