I've been thinking just removing the MP / Blood from TBN and just let it be a defensive cooldown. It's not like paladins really give up much for using Shelltron, and many have highlighted this as primarily because the Oath gauge only powers a subset of abilities, and is not really a universal resource in the way MP is.

Remove the cost and blood conversion, give it a longer cooldown, and make the shield somewhat stronger. Something like 30% / 30 seconds. A longer cooldown discourages using it for everything, and a stronger shield makes it more likely to be impactful as a standalone cooldown.

We can take it a step further and empower it under Blood Weapon and Blood Price

TBN Breaks under Blood Price: Gain 50 Blood.
TBN Breaks under Blood Weapon: Heal for X Health.