May be adding some sound when there's aggro loss would make thing easier for some ?
Or adding that little red circle next to the mob's floating health bar ? Something like the threatplate addon ?

As many others said, on a PLD, threat isn't a problem at all, on a dungeon boss fight you can easily keep aggro with just a roh combo and spend the rest of the fight on sword oath.
And from what I've played with other tanks, I don't think they should have multi aggro problem (unleash and dominate give so much threat...)

once you have the mindset of "spam flash"
I can confirm that was hard for me... especially when the other two tanks have such flashy moves like unleash and dominate !
Not to mention the animation and the sound of flash are so uncool !
I felt the same with holy spirit : I hate that part of the rotation, feels so dumb to be a caster with swords and shield :/