i'm an healer (resto druid) and i killed mythic mar'gok few days ago
I'm also in charge of handling p2 debuff, so i know how frustrating can be for an healer to top off arcanewraths while moving and healing high priority targets.
While looking at your gif isn't "that" informative, i can see ofc an issue healing branded but hey, at the same time you could have saved yourself easily: if you have 3rd stack and you aren't healed between 3rd and 4th, just fuckin move and lose a global w/o risking to die.
Ok it sounds stupid for most ppl, but the key of the fight (for me and my guild btw) was to not to die in early fight and save resses for key phases (high dmg in interphase 2 e p3 for example).
in the end, i agree with ppl saying healing has changed blah blah...it's Dps, tanks and healer job to get used to change
good luck with the progression!
cheers
p.s. sorry for the bad english ofc i'm not mothertongue and too lazy and tired to check it