uhh with the new 1/1 trait healing rain isn't getting competed out by high tide chain heals anymore, so the boots are actually pretty good and I've been keeping down rain a lot more
but I generally agree with your current setups
but uhhh for the first 7 mythic bosses I'd say
spellblade (legs assumed here): if you have trouble with keeping people up through frost: belt, if you're having trouble with the burst healing of arcane: (probably shouldn't be too bad if you have link+darkness), use boots
and lusting on pull, fire, or arcane adds are all viable and might affect the value of the helmet
Unsure if it's been fixed yet, the part where you hero > change helmet off and keep the extended heroism buff still? If it does, you might want to try that, so you can get another legendary in for the fight instead of holding onto the helmet for the rest of the fight.
Sephuz wouldn't really benefit our class as much, but if your group likes to stand in healing rain + u don't mind having lots of haste, it'll be good. Don't forget to proc it through interrupt / stun / dispel / purge. Its also a very good legendary if your tank/group is reliable enough to give you time to help dps stuff
The boots might be good too for the above scenario, but always ensure your leggs are on. (Unless volcanic like this week, you'll be moving way too much)
Raids: ryklin's suggestion is good. If your group uses heroism on the arcane adds, may suggest leggs + helmet. Having that heroism buff up for the frost phase 2 may help your group loads while the rest of your healer team drinks mp / saves mp. Do let them know beforehand though so they don't overheal while you go ham.
Stack up or you get no heals! || Water control perfect, still can't attack with it
Signature by Winter Blossom
Whats the benefit of using Darkness on top of SLT? Seems like a waste of a cooldown with how strong SLT is by itself?
On fights like Chronomatic where you're taking a heavy part of your HP at one time, the overlap can save lives. If a few people don't get hit by power overwhelming, that pulls the health of everyone in the raid up, essentially reducing how much healing you need to prepare for the next hit.
For mythic progression I'm currently swapping between bl helm and tidecallers whilst always using future sight. Although jonats is also pretty strong depending on your play style and it you prefer some more crit. Currently 7/10 and starbro should be down this week. The trinket is insane for resto shammys. Especially combining it with a healing tide. I'm trying to using bl helm instead of tidecallers on starbro because we are bloodlusting the fel phase. Where everyone takes insane amount of damage and the 25% healing increase is amazing to help keep everyone up. I find tidecallers are great but if you really need more healing for a high intake damage period use the bl helm.