It remains that the calendar question is a theological concern
Your position is at odds with St. Athanasius' account of Nicea (the only Ecumenical Council to make any statement about anything calendar-related). In his 'De Synodis' he writes:
"Without pre-fixing Consulate, month, and day, they [the Fathers of the 1st Ecumenical Council] wrote concerning Easter, ‘It seemed good as follows,’ for it did then seem good that there should be a general compliance; but about the faith they wrote not, ‘It seemed good,’ but, ‘Thus believes the Catholic Church;’"
In other words he carefully distinguishes between Nicea's decision about 'the faith' (i.e., Christ's divinity), and the decision about Pascha which was not 'about the faith' but merely a matter of good order.
A few other notes since there is a lot misinformation about what Nicea actually said about Pascha:
1) Nicea did not assert the Julian Calendar. In fact, Nicea did not mention any calendar or date. Nicea mentioned exactly 2 things - the vernal equinox (an astronomical event) and 'the Jewish passover' (which is based on a lunar calendar and will therefore always move around in relation to solar calendars like the Julian and Gregorian).
2) Nicea did not establish a specific Paschalion - it established certain standards with regards to the vernal equinox and the Jewish Passover that any Paschalion needs to follow, but Rome, for example, used 3 different 'Nicean' Paschalions between Nicea (315 AD) and 525AD when they finally settled on using the same one as Alexandria was using. The "Paschal controversy" you can find in any account of the history of Christianity in Britain arose because the Celtic Church was using one of Rome's old Paschalion's which did not always coincide with the one Rome was using by the 7th and 8th centuries--but both the Celtic and Roman/Alexandrian Paschalion's met the criteria of Nicea.
3) In the Paschal calculations produced by the Church in the centuries immediately following Nicea, the official Julian calendar date for the vernal equinox (March 25th) was not used. Instead, they use March 21st which was the date the actual astronomical event was occuring on at that point (since the Julian calendar was already off solar reality by 4 days at the time).