Honestly this explains a lot of why you’re saying what you’re saying.
If you have an organized group of familiar people whose playstyle you know well and can trust not to stand in the bad, you take so much less damage overall it’s not even funny. You really can just leave people at 80-85% (barring Grievous) and top them off between pulls.
If you pug everything, every week is effectively a Grievous week and you’re going to be grabbing for any extra HPS you can get.
Every once in a while last season our main M+ group tagged random people into 15s who didn’t have a weekly 10 yet. I healed less in those 15s than a typical pug 7.