The idea with bursting in general, not just holy priests, is to focus people as much as possible to keep stacks low or stacks-free. In emergencies, you use mass dispel and pat yourself on the back.
For general play in high stacks situation if dps are too stupid to avoid 6 stacks: Prayer of mending before heavy stacks, not during, its a good spell but its more about long term healing and not immediate savig. Sanctuary the entire team (or atleast 3 people), Serenity someone, activate apothoesis, repeat those two holy words, then spam flash heal. the Heal-buffing legendary (which makes Heal much faster and stronger then Flash Heal) helps a ton here and in general is great for holy priest, but if you dont have it just spam flash heal. Avoid prayer of mending it doesnt heal enough per cast time to save people in heavy bursting, and consider avoiding circle of healing if someone will die in two gcds.
The idea is, spam your biggest hitting heals in triage and use over time stuff before damage gets bad and not after (so if you Divine Hymn, do it when stacks are starting to apply and before people are super low, that will help people stabilise without endagering anyone whereas if you do it late someone might die). Good luck and have fun, i personally enjoy grevious weeks because they give me a reason to actually heal the entire group often rather than just put the ocasional heal and dps, which is fun healing practice.