Try out the High Tide talent on the level 100 row when you are casting Chain Heal frequently. I find Wellspring more situational while High Tide is my go to for a Chain Heal fight. It heals for so much more and it’s solid increase in the power of your chain heal. Look at Vixxi’s spell balance on the Demonic Inquisition fight.
Your Critical Strike is very low. When your heals crit not only do you get double heals but you also get mana back in return. I would love to get more crit myself but I haven’t had the right pieces drop. I’d suggest switching your enchants from Mastery to Crit and try switch in more crit when you have gear available. This will give you the mana returns you need for the long fights. Your neck, ring, shoulder and waist are slots you could prioritize without sacrificing your 4 piece bonus.
Just remember you are running with other group healers so if you want to pump more heals on the logs look at timing your spells so they land just after the damage occurs. If you are merely topping off the group then your Mastery is not doing much for you but when your spells land on low health targets you heal for a good deal more. Always prioritize the lowest health target as the person to cast your heals on.
My group has just started on Fallen Avatar and your current healing style is actually very good for it so keep this in mind since the group is really spread out which limits the Chain Heal/Healing Rain combo and you need to cast more single target heals. From our first few attempts it does not look like a Resto Shaman friendly fight.
Other than that it looks like you have a really good balance in your spell rotation so it’s more gear/talent change which will make the difference for you.