I will add here the answers to questions that I've been asked.
Q: What add-ons are you using?
Q: Why don't you use the ritual mundus with full magicka recovery glyphs and or clockwork citrus filet, instead of the atronach with spell damage glyphs and blue food?
A: The ritual mundus does provide for a bit of more healing done, but is, in general not needed. On the other hand, the use of blue food and spell damage glyphs also boosts your off-dps which is more important in the current state of the game.
Q: Why tristats glyphs over max magicka glyphs and more attribute points in health?
A: While this would give you the same amount of health and magicka, it would make you lose some precious stamina. Stamina on a healer is especially important : not only blocking/not blocking can be the difference between life and death, but you cannot afford to stay stunned because you don't have stamina to break free, for example.
Q: Healing Springs or Illustrious Healing?
A: Healing Springs provides for a huge boost to your sustain, as it can recover up to ~900 magicka per cast. However, I've been testing out Illustrious Healing lately, and its longer duration enables you to buff a bit more your group. As it is really a big change for your sustain, I would advise to use it only with an Asylum Restoration Staff (to enjoy the 30% cost reduction on cast), and only once you're comfortable enough with the trial you are about to do. If you are not able to give enough support to your group – in particular, orbs or shards – because you can't sustain, there is no point in using Illustrious Healing, even if you saw a healer from a good group using them.