I’m just a DPS that does ~22-23s casually with friends so I don’t have the best idea of what being a healer is like, but I feel like if you’re needing fast reactions in trash pulls etc your party is the problem, not you. It’s likely that at the +17 pug level you’re in groups where people are doing ~5 kicks per dungeon, no one is using stuns to stop extra mechanics/kicks, people are standing in mechanics, no one uses defensives against spike damage etc. There are definitely some stressful healing bosses this season as well, made somehow worse by the fact that they usually don’t have any real mechanics going on - just a straight healer check.

My only advice is to group with friends in voice and learn what’s dangerous in the trash pulls and stop or counter it. Group with rational party comps that have enough kicks, stuns, aoe stuns, and defensives. Get an addon called ElitismHelper and configure it to print to your chat frame (by default I think it prints in party chat?). 100% of the time, when someone says “healer wtf” I can look and see “bob took 400k failure damage” in my chat log. Get an automarker weakaura so that all kickable/stunnable trash has a raid marker above it, then have everyone in your party call out a target marker and be responsible for kicking/stunning it. Get an interrupt tracking addon (e.g. OmniCD) and be aware of who has kicks available for casts. Tell everyone in your party to put their kick on CD 100% of the time. They don’t need to be kicking the right casts, they need to be kicking as soon as their kick comes up, every time. Once they’re in that habit, they can figure out what spells are actually dangerous.

It’s really night and day playing with a group that plays dungeons with their full toolkit. Try watching a high level key to see how little damage they take. Many of my healer friends say that healing low keys is actually more stressful than high keys.