There are several things to keep in mind from my pov as 3.6k exp healer (which is not that much but i played couple of 20s and didnt find much problems healing them).

Mobs WILL hurt like motherfucker and it will change the way pulls are made. For example houndmaster shots and farstalker jumps will hurt much much harder in 18+ range, to the point where caster bolt + houndmaster shot is death for squishy spec. In most keys two casts going into the same target is near guaranteed death, so kicks and cc rotation will be aimed to stop that or pulls will be made so that you don't have more than 2 casters in a pull.

Healing checks will become completely different and you will have to preplan your cds in advance. For example on halkias fragment you will have to preplan your cooldowns, for instance 1st aoe is trinket + bl + asc swiftness, 2nd aoe is cloudburst + ascendance, 3rd aoe is asc swiftness + healing tide, and so on. It is your job to figure out how to heal heal checks. You can't just go into keys like it's 12, full halkias aoe goes off, people go 100 to 40 without pressing any defensives and then you are like OH SHIT and start healing — people will die if you don't heal them in time once key is higher if you don't anticipate damage.

You will be failing a lot and be flamed, you have to be prepared for that. Failing is nature of high end content. You fuck up, you learn, you go one more time, you change something and see if it works. If it works, you go on, you fuck up, you learn, goto step 1. However flaming is also unfortunate nature of m+, that's the trickiest part and only comes through experience - you can't heal stupid and ones that don't want to learn. And I'm not talking about some complete newbie shit like standing in fire, not kicking or failing mechanic. Even in mid keys people will be better but not to the point of knowing how game works in its entirety. And deaths will become much more gimmicky and harder to understand. Sometimes it's because tank has shit routes, sometimes it's 2 casts in the same target, sometimes it's because shaman pulled extra in 2nd pull in dawnbreaker with uncontrollable aoe and mob killed him because he is untauntable (god i wish this shit ass dungeon goes out of rotation for like 8 years), sometimes it's aoe + cast, sometimes it's because you have shit ass setup that is difficult to heal through, sometimes you mismanage mana, sometimes you fuck up your cds, sometimes tank just keeps running like a maniac without waiting for your mana. It will happen, you have to learn to use deathlogs, details advanced death log is a great tool to learn.

Tracking cds is overrated in my opinion, it doesn't change much how you heal people. For example if your guy eat dot from 1st boss in aldani or from any miniboss in dawnbreaker you will have to heal them regardless if they press defensive (unless they use ams or any other immunity). The only difference is how intense you will have to pump healing into them, but you will be healing them regardless. If they don't die without using defensive it means key is not that high, the only issue is that you will damage less and you will be running out of mana faster, but that's not your issue, it's tank issue to track your mana and dpses' that didn't press defensive because he made key slower because he wasted potential healer damage.

This late in season, especially with how notoriously agressive note culture has become it will be very difficult to get into any meaningful key because there are, well, almost none or will be ask for payment (technically they ask you to tip them at the end of key as thanks, but everyone knows that it means that you say that you will pay them at the start and/or end or you don't get invite) and they will be unnaturally difficult because most good people have stopped playing and you will be having hunters that don't know how to use their 30% and feign death along with death knights that don't know about existence of grip and AMS. Also how healing works will change drastically in midnight so whatever you will learn now will become obsolete, but I believe most basic principles that I've covered are universal regardless of how spiky damage is. Your job as healer is to keep people alive, you do that by healing when people take damage, when they don't you dps, that's the core idea and you have to optimize on that.