It’s 100% a one-time buff that modifies the next move. You can see it in the job action trailer as it has its own UI to indicate when Eukrasia is active or not and it disappears after you use Dosis, Diagnosis, or Prognosis.

It’s a reversal of Emergency Tactics. Emergency Tactics converts your next heal spell from a heal/shield into a heal. Eukrasia converts your next heal into a heal/shield or your next nuke into a DoT.

But I do agree that SGE’s GCD style is closer to Noct AST than SCH.