I’m not gonna analyse your logs, I’m not a huge fan of saying you need 100% up-time on this and that. Here are some things I’ve noticed about shaman healing in Emerald Nightmare and as a ex-holy paladin.
Strats matter: Heroic is generally a free for all for me, but when it comes to mythic things buckle down and get set in motion. Cds are planned, positioning is organised, and you plan and prepare based on what your team is doing. Chain heal and even healing rain can under perform if your group isn’t positioning well for it, think stack and spread fights.
Situational healing: sometimes I delay a healing rain because a few people are just too low to ignore then to keep my 100% healing rain up-time. Healing is always slightly different in each attempt or kill except for when and how you use cds.
Assignments: does the team expect you to heal only the raid or help the tank too. Are there priority targets for mechanics. That will also change your spell choices, talent choices and etc.
So far for me in Emerald Nightmare I haven’t defaulted into the typical chain heal + cloudburst spec at all (not until Mythic Xavius). I still dislike cloudburst and find echo’s increase to Riptide and Tidal Waves far too useful. Cloudburst has its place on Ursoc sure. I got Mythic Cenarius last night and I used Echo, Crashing Waves, Ascendance, Torrent, and Gust of Wind because it is what worked for ME. I even tried to be a chain heal shaman for a few attempts and people died!
So point is, experiment and think about what your role is in the fight. If the big damage moments are covered by cds (like feeding time on Elerethe) chain heal is too slow and will probably over heal, using some spot healing to save those that are lowest while a tranq or tide is up can be far more useful to your team. Alternatively, on a fight that has predictable aoe moments like Mythic Xavius or Ursoc you can pre cast chain heal knowing the other healers have the spot healing covered. Timing cloudburst for the big damage hits, and trying to overheal before hand to feed it with heals… I just don’t like that mentality.
Anyway, don’t take what I say as “this is how you must do it” just see if you get any ideas, and good luck.