While this is more of a band-aid on the wound than an actual solution (healer metaphors!), I will note that you can bind the mouse wheel, along with various modifier keys, to shifting among targets. I have the normal mouse wheel bound to the target list, and then Shift plus the mouse wheel bound to the party list. (Alt + mouse wheel is bound to the alliance list for alliance raids, though I've found I don't use it often; Control + the mouse wheel is bound to the camera zoom in/out functionality.)

It took a bit of getting used to, but I can now move exactly where I intend to in the target or party list with just a quick twist of the mouse wheel, which is very useful for selecting things (without having to reach up and hit F1-F8 to select party members that way). And as an added benefit, in raids where the boss is large enough that tab-targeting won't always select them again right away, the mouse-wheel-through-aggro-list method will always select them, making it much easier to consistently select the boss again after healing someone.

I also agree that improvements to how mouseover targeting for actions work would be hugely beneficial to healers in general, mind you, though the macro system as a whole is kinda broken and I suspect they'd need to take another route (like a toggle for 'apply action to friendly targets on mouseover' and then just using the action outright) unless they just torched the macro system entirely and wrote something new. I just offer this note up in case it's useful to anyone else in the meantime.