I gotta be honest - the more and more I see this issue, I question if really the initial insert of the Filioque was really that bad or insinuated heresy. I think it only became heretical when Charlemagne and the Holy Roman Empire began accusing the Orthodox of heresy by NOT having the Filioque and the Roman Church began believing the Filioque applied to the Son as a cause / Eternal relations with the Holy Spirit, rather than just the temporal sending.
Many people would argue that a change of the original Nicene Creed is uncanonical, but I find that argument superfluous because the version the Eastern Orthodox use IS different - it is changed from the 2nd person to the 1st person. Moreover, there exists non-Chalcedonian versions of the Nicene Creed which have minor changes, and nobody, NOBODY complains about the "God from God" insertion before the "Light from light" in the Roman Catholic version of the Nicene Creed.
What is uncanonical is changing the Faith of the Nicene Creed - and the question is, does this constitute such a grave change in the Faith of the Nicene Creed?
Some will argue that the context of the Greek term of "proceeds out of" had a specific connotation of causation, therefore the Filioque implies heresy - but we have to remind ourselves that starting from the 4th century onwards, Rome started exclusively using Latin instead of Greek (which led to Augustinian problems, but that's for another time); but even then, even in the original Greek - we have to remind ourselves - specific Greek terminology can mean different things at different times; define what Saint Cyril means by "Physis," because "physis" can mean both "ousia" and "hypostasis" depending on the context. Likewise, it MUST be the case that the verb for "proceeds out of" didn't always have such a specific connotation in all contexts, if Orthodoxy is True, because Saint John in the Book of Revelation uses the EXACT SAME GREEK VERB in Revelation 22:1, with "water proceeding out of the Throne AND OUT OF THE LAMB (the Holy Spirit proceeding from the Father and the Son)." I don't think it refers here to eternal relations, but the act of the renewed creation, with the renewal of the City of Jerusalem at the end of the world.
Saint Maximus the Confessor was actually asked about the Filioque when he sought refuge in Orthodox Rome, and he explained that as far as he understood it, it was Orthodox because the Latins make it clear in their own Latin tongue that it refers to temporal procession and it doesn't make the Son a cause of the Holy Spirit.
Of course, as Florence dogmatically proclaimed, the Son is "indeed a principle, to the Greeks as a cause of the Holy Spirit", so as the Filioque is understood now, it definitely is heretical, but I don't think it was heretical given the fact that it was understood in Latin that the Filioque referred to Temporal procession.
Those are my two cents, and if I'm wrong, feel free to correct me.