So that last limit break is all about how useful the upgrade to their 2nd skill is. Let's go over each one:
Parvati - 'performance up'; presumably an increase to the amount of the atk/ability damage buff, but wiki doesn't say how much
Hermes - increases duration from 1 to 2 turns
Cronus - the atk buff aspect gets increased; unlisted amount
Maeve - doubles the reflection damage. The skill originally reflects back 1.5 times the received damage, so I think this upgrade changes the multiplier from 1.5x to 2x. Going from 1.5x to 3x seems a bit beyond what I'd see from skill upgrades for SR.
Freja - atk/def buff increased
Krampus - rage gauge reduction upped from ~10% to ~25% (wait, that seems rather high)
Ithaqua - heal amount increased from 800 to 1000
Nemesis - 3.5-4.0x damage against raging enemies becomes 4.0x
Psyche - heal amount increased from 800 to 1200
Tsukuyomi - reduce cooldown by 1 turn
Pharol - reduce cooldown by 1 turn
Lu Bu - debuff amount raised from 10% to 15%
Meretseger - reduce cooldown by 1 turn
If you don't have the dark variant of Amaterasu, then limit break Lu Bu.
Aside from her, it gets harder to say without knowing what else you have/who you actually intend to use.
In a vacuum, Hermes/Ithaqua/Krampus (if that 25% is right) have the next most impactful skill upgrades.
What I'd do is just limit break Lu Bu and save the grimoires for the time being. But I also tend to only carry out these last limit breaks for the SRs I intend on using in the long term (barring lucky SSR rolls).