You maintained FC stacks very well. The major thing I would work on is weaving in damage. Smite is your filler when there is no healing needed and it helps conserve your mana to fish for surge of light procs. You did no damage at all really. And just like you’d expect dps to use their personals and heal themselves up when able, you should expect your healers to assist in killing the boss during down time as well.
Another few minor things are power word shield usage, renew, and lack of desperate prayer. Power word shield and renew are high cost spells that have very niche uses. I wouldn’t even really bother using those on anyone unless you have literally nothing else to cast and someone is about to die.
And then the desperate prayer thing. Sylvanas is a long fight and you could have easily cast about 6 more desperate prayers. And actually when you died you probably could have saved yourself if you pressed that ability before the second wailing arrow hit.
As far as being necro or whatever, it’s not going to change your healing numbers significantly. I play both covenants and while the extra mana from night fae is nice, it can be wasted on farm reclears when mana is no longer an issue. There are benefits for either and it really just depends on where your group is at.
Do you have a way to track when your holy words are off cool down? That can help boost your numbers as well. I wouldn’t sit on your holy words for too long, unless you know a damage event is going to happen 5-10 secs later and it’d be better suited to wait and use then.