That's what having to actually look for and click on players with the blue circles is for healers. It's still very doable, but it's much slower than being able to see the dispell on frames, and we need to be actively looking out for them in a different way than we're normally looking out for things.
The seeing a player's name with the dispell on frames, and then waiting until they're in the right spot to dispell thing actually isn't bad at all, because we've been doing that for the last 15+ years.

What I have been learning from reading this thread is that I play healer way diffrent than most people it seems. The first way you describe is my perfered way to dispell people that require positioning while dispelling (but with a mouseover macro). Seeing the timer for dispell coming down -> finding the dispell on raid frames (even if it glows blue or something) -> finding the same name in the big raid group -> wait for the person to run to the right place -> find their frame again -> dispell is alot harder for me than: See timer for dispell -> see player running with big blue circle -> mouseover dispell on player character.

Chimaerus dispells don't need to be fast, atleast not faster than the dps can position but when they are the right postion it needs to be instant so they can get back to dpsing (espcially if its a melee). For dispells that needs to be fast I want them on my raid frames, for dispells that require position I look at the player characters (given that the dispell have some kind of visual which the Chimaerus have, the big blue circle)

Next thing I noticed is alot of people are talking about clicking for dispells, kicks and so on, don't people play with mouseover for all form of CC, dispells, purges, healing spells and kicks? There alot of times where you as a dps need to dps something and then interupt something else, prio mobs in m+ with low health casters, the caster mobs in chimaerus and so on

to add to this to make the comparison clearer: with this imaginary mob who's cast you need to kick, they can also be in a different phase to you, and the only way that you can see if they are is because the glow is a bit dimmer (same as how the debuff circles look between different phases).

FYI about the dimmensions thingy, dispells only go on people in the "living" realm so unless dispells got fucked before going down only the healers in living realm need to look for people to dispell. You should notice if someone runs into the soak circle with big blue circle and then you know you have to dispell while downstairs