Sanctify:
Releases miraculous light at a target location, healing up to 5 allies within 10 yds for (357.93% of Spell power).
Instantly cast Heal on your most injured ally at 65% effectiveness, then release a miraculous light which heals 4 more of your most injured allies for the same amount.
Making Sanctify rely on Heal ties Holy’s single target healing and aoe healing together. this enables a more streamline design where buffing your Heal, which the spec heavily relies upon ever since Shadowlands, also contribute to your aoe healing.
Further, this fixes the inability of Holy Priest to reliably heal frequent aoe damage instances in m+, while also creating room to maximize your raid output by more sophisticated gameplay, making it more interesting for advanced players, while also not too punishing for new ones.
The innate cooldown of Sanctify as a big rewarding spell which is available only once in a while, fits perfectly for the spots where Holy struggles the most. Big aoe heal on a ~20 seconds cooldown is exactly what it lacks in m+, while buffing it will not create too big of a difference in overall hps in raids (where holy could also use a minor buff).