You illustrate the mistake of looking at the reasoning for something from only one view, which is exactly what happens when you rely on the early theologians of your church.
I picked the Donatist's for a very specific reason. They were a part of your church and left for very specific reasons. They were not considered "heretics" until they refused to submit to a central authority that was backed by the power of the state.
AS to whether they were considered heretics, what do you think Augie thought of them? Surely he thought they were in error. I'd guess they couldn't really rise to the level of obstinacy required for heresy until there was an authoritative statement to the effect of, "Yo! Y'all are WRONG! Chill!"
Is that the reaspon St. Augustine gave, or is it an assertion of your own?
On your larger point, of course the fathers of the Chruch are important in clarifying the position of the Church, including the position vis-a-vis the heretics. That's news?
They were heretics from the beginning because they did not teach what the Church taught. The is the definition of heresy. Only Ecumenical Councils can call someone a heretic. But, then, only Ecumenical Councils can change the Creed too. It was also the first Ecumenical Council that decreed there would be no kneeling on Sundays. Obviously this never stopped the Church in the west, which acted on its own, often disregarding the binding nature of Ecumenical Councils.
As far as Donatists are concerned, they were no different than Cromwell's Puritans—they represented an extreme minority. Specifically, they rejected the absolute forgiveness for repentence professed in the Gospels. Doesn't get much more heretical than that!
Donatists were what you would call extremes, ultra-con-servatives. You have similar examples among Russia's Old-Believers and various other puritanical sects all over Christendom.
Once they denied the Sacraments of the Church, they exocmmunicated themselves from the Church.