For Limit Break 1 it would restore 50% Health to the party and give the healer who used it full MP. I've had to use it enough times to know that it's only because I'm desperate to heal and out of MP at the time. I would want it to be a smart heal though, in that people at full health would be ignored in favor of giving more health to those lacking. So if the tank died and recently revived in a dungeon and the party has just started taking damage a healer LB1 would devote most of the healing potency to the tank bringing them to full and topping off everyone else. In a raid or trial if everyone is low it would just be 50% more health for everyone. This would make the increased health pool and reduced damage of tanks more valuable as the Healer Limit Break would funnel the healing they don't need to the party. If you need everyone topped off for a mechanic and only half the party need health then a LB1 is suddenly a legit option to save a scuffed run.

For Limit Break 2 I would have it restore 75% health to everyone and restore MP to full for all Healers in the party. That's close enough to full that you could definitely get most people through a heavy hitting mechanic with it. It would be a nice way for a solo healer to resolve a dropped to 1 hp mechanic but everyone needs health quickly.

I would also make it so that Healer Limit Break 1 and 2 can't be interrupted and the casts completes even if the healer would have died mid mechanic. An invulnerable state while in the casting animation would be some great tech to play around especially when stack markers almost exclusively target healers. I would only apply the invulnerable state to Limit Break 1 and 2 tho as using it early to cheese a mechanic would have the trade off of losing Limit Break 3 later on which is so powerful that an invulnerable state would break the game imo.

Healer Limit Breaks that actually have a sizable amount of smart healing (meaning healing that prioritizes low members first) would make me use the button a lot more especially if timing them well guaranteed my own survival too.