I don’t heal, so I’m pretty useless for advice, but I can suggest a couple of things:
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For healers specifically, Proving Grounds is a great way to set up your addons/macros and get comfortable with them. They’re a lot easier than they used to be, but if you can get Gold you have mastered your basic mechanics. You seem to have done that already?
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Of course doing lower content like HCs will ease you into it more smoothly. I have tried healing enough to know that the way you look at the dynamics of a group, and how it moves through a dungeon, is very different from DPS, so doing a few that way would certainly get you into the mindset.
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There are communities that form groups for M+. You might try Calm Keystones [H] Mythics - Calm Keystones or Fail Train https://discord.gg/VsEFRHh to find friendly groups for low M+ if you don’t get them with your guild.