Eventhough Healer units are a thing since the release of the original game, they never managed to become as effective to be used in competitive games, it would be beneficial to make adjustment for the Medicine/healer units overall in order to put them into a usable spot.
What could be done?
I mean, Aizamk used the priests in some games. But he also play Russia, so maybe “competitive” might be a strong word. 
Honestly, the only opinion I have on this topic is that the healers where better on the legacy version, where you have a ability that you have to use for them to heal, but they used to heal units really fast. I didn’t like the Asian Dynasties version where they heal automatically and much slower, I think is a much worst version of those units
They clearly have implemented passive timed attacks so they can also implemented an automatic timed burst healing effect for units at 50 percent or less and allow the ability to function DURING combat/movement of hurt unit(s).
Spain monks should have unction as a church upgrade instead of a home shipment card.
Lol Aizamk will be aizamk, always doing some wierd off meta strategies
I feel like healers should heal ALL nearby units at a very slow rate mid battle but have a build limit
It is hard to agree with this statement, because what is stated in the op is explaining the healers not becoming as effective to be used in competitive games, which is not an opinion but a fact in reality. Showing spesific player/name as an example to make the statement irrelevant is not helping any for the discussion.
Hum. I was memeing though. 
I think that healer units should just heal in an aura, one unit at a time is just simply too slow in a game where you constantly want to apply pressure, but keep the fact that it can only work when out of combat. If it was just a bit quicker then it could actually be viable and open up some tactical decisions, like 'do i continue pushing? or take about 15~ secs to heal up my army before the next push.
Agree, that would be quite beneficial.
You said it best. Either they inteoduce combat healing or they speed up out of combat healing because at high levels there is many many pauses before major clashes.