Positioning has so many variables like you say because the pace changes.
It’s also going to depend on map, your team comp, and the enemy’s team comp.
Usually you hang back with Ana while your support protects you. Like that’s a default start position.
At the same time depending on the flow of the fight you want to be close enough to accurately throw nade to save your front line or punish the Frontline if the tanks barrier breaks.
There’s not a set rule because your team nor your hero want to get predictable in a position unless the enemy can’t break your defense.
You have to have your head on a swivel as a support so also have yourself positioned near escape routes where you can corner peak from mid range/snipers trying to poke you.
Sometimes you gotta move forward aggressively with your team, some times you can’t risk it especially if the enemy is a good shot.
But you are one of the all-star pieces because that made can roll the enemy if you are in the right spot to splash them with purple.
It’s a lot of layers because you gotta figure out do you need to position by ceiling cover to not get jumped by Pharah/Echo.
Should you play far back or stay up close to make an enemy Ram think twice about ulting or risk you putting him to sleep.
Positioning and spacing is always changing in the game.
But to keep it simple, put yourself in the best positions to heal but give yourself the least exposure to enemy fire.
You are slow and diveable. And primary target of a dive tank like Dva and flankers.