Suggestion - Increase PLD Healing Support, not Dps

[Reynhart]

Thus why the only way for this to work would be to make PLD an actual healer.

Not necessary an "actual" healer. For example, if you can only heal a single target, that doesn't make you a real healer.
Or you could adjust PLD healing capacity to be specifically designed to heal the main tank. Like varying potency based on the target's enmity.

PLD could also help others, not by healing but by offering more mitigation. We could have cover on a shorter cooldown, or Sheltron that could block the next hit as long as the target is in range (The range of the animation).
Which would help you replenish some MP to keep healing.

With that, a PLD OT could delay the HP loss of the main tank, allowing the real main healer to focus the whole party when needed.

Give Sword Oath a raw DPS boost even 5% would be something

Sword Oath should increase potency of all actions by 20%. With our auto-attack speed, the effect would be slightly less than what it is now, BUT, it would benefit the enmity gain of weapon skills, for those would like to tank in Sword Oath once in a while, AND, would also boost the potencies of oGCD skill, like Circle Of Scorn and Spirits Within, AND, more importantly for this topic, would also boost the potency of Clemency, thus making it more viable to keep the main tank alive.