So learning Ruin III at 54 is actively harmful to the game.
The spell hits for 1.5x potency and costs over 6 times the MP when compared to Ruin 1. And let's be real, that's still 120 potency, which is less than the second attack in a three-stage melee combo.
So it's hideously inefficient outside of Dreadwyrm Trance. So much so that the only reason to cast it outside of Dreadwyrm Trance is to see the animation. At least it's pretty.
The best thing to do with the spell you learn at 54, is to keep it off your bars until 58. Which is really not great. "Here's your reward, it's really bad for the next 4 levels!"
The skill order should really be
52 - Painflare
54 - Tri-disaster
56 - Dreadwyrm Trance
58 - Ruin III
60 - Deathflare