I have tried to convey very firmly that what was settled on was not invented at Nicea ...
OK. It's me then, drawing the wrong conclusion.
The only caveat is that it seems much more than mere coincidence that beginning at Nicea, and apparently rather blatantly at Laodicea about 40 years later the separation from anything at all to do with Judaism was codified and institutionalized. Constantine, being the benefactor and simultaneously being openly and vocally anti-Semetic would appear, rightly or wrongly, to have some degree of influence on the decision to separate the newly minted religion from its Judaic roots.
Maybe. The roots certainly go back though, it's not all his fault. There were running street-fights between Jews and Christians in Rome in 60AD. And then the Christians were banned from attending synagogue in 70AD.
I read somewhere that when Jerusalem got wind of the Roman decision to flatten the place, the Christians got out, so the Jews would see that in a poor light, too.
So whether Constantine's anti-semitism played into it or not I don't know, but there was certainly enough of it going round anyway, I think. Give it a few generations, by which time the gentile percentage of Christians was well into the ascendant, and tribalism will rear its head.
But to suggest all was well in the Christian world prior to Nicea is to be wearing rose colored glasses, when we both know full well the bickering yet continued, frankly to this very day!
Oh sure ... and in hindsight, in responding to you I was probably unloading on all those who insist Constantine invented Christianity, wrote the NT, wrote the Creed, told the ftahers what to think, etc., etc ... so apologies for that. I do get a bit 'patristic' in my arguments sometimes

Thank God it's just me ... Tertullian, a skilled rhetoritician, would not desist until he had destroyed his opponent's argument, his character, his family, his friends

And Athanasius was not unknown for popping round with some of the brawnier brothers to sort out a dispute ...
I had a long conversation with a Dominican theologian once, who's line was, 'go easy on Arius, he wasn't all bad, and just did what he thought was right'.
As for the rest, I'm not even going to validate this juggling of calendars with a response.
Don't have to, it's not 'juggling', it's seemly accounting for what calendars were being used by whom.
The quest was for celebratory "feast" as you put it, recognizing the birth of Messiah prior to Nicea.
Well we have the dates, I have no idea what kind of feast was celebrated, although the tradition of Jan 6 seems stronger in the East, tied in with the Birth, the Baptism, the coming of the Magi, everything seems to have got lumped in.
My contention has only ever been twofold:
1: There is no evidence nor reason to suppose the Dec 25 date was arrived at through syncretism or assimilation.
The point is Nicea *began* the official, dogmatic, doctrinal separation from Messiah's native Judaism, and Christmas is a token example of that separation.
A hard claim to make stick, I would have thought, on the basis that none of the Nicean documents say anything about Judaism, nor was Judaism discussed, as far as we know.
Personally I think the Church was a long way from its Jewish roots by 325.
The Lord's Day is mentioned in Ignatius'
Letter to the Magnesians mentions observing the Lord's day rather than the Sabbath in 110AD and
The Letter of Barnabas mentions the first day of the week in opposition to the Jewish Sabbath around 130AD, so there's a marked distance between Jewish and Christian custom centuries before Nicea, and Gentile converts were not obliged to observe any Jewish customs as we can see in Acts.
A couple of points of interest in the Canons:
Canon 5:
"... And let these synods be held, the one before Lent ..."
Lent was a very early practice. Irenaeus mentions differences in various churches over how long they fasted prior to Passover in 185AD (Eusebius,
Ecclesiastical History V:24). So there's a church calendar in place.
Canon 6:
"... the Bishop of Alexandria have jurisdiction in all these (local area), since the like is customary for the Bishop of Rome also. Likewise in Antioch and the other provinces, let the Churches retain their privileges..."
The emergence of Rome, Alexandra and Antioch as principle Patriarchies.
Canon 7:
"Since custom and ancient tradition have prevailed that the Bishop of Ælia (Jerusalem) should be honoured, let him, saving its due dignity to the Metropolis, have the next place of honour."
So Jerusalem has an 'honoured' position for historical reasons, but not equal to the above three. That can be read as marking the separation from its Jewish roots, to be sure.
In the years that followed, Constantinople assumed a Patriarch status, being the seat of Constantine. It usurped the authority of Jerusalem, challenged Alexandra and Antioch, and was obviously intent on seeing itself at least as the equal of Rome. This politicking sometimes played into theological discussion.
Canon 11:
"Concerning those who have fallen ... As many as were communicants, if they heartily repent, shall pass three years among the hearers; for seven years they shall be prostrators; and for two years they shall communicate with the people in prayers, but without oblation."
Church discipline: In 4th century congregations, there were those admitted to communion; there were those learning the basics of the faith in preparation for baptism (
catechumen) there were hearers, and there were prostrators (penitents), who were forbidden from communion in penance for something they did.
This canon requires those who lapsed had to wait 12 years before they could receive the Eucharist again!
Canon 12:
"As many as were called by grace, and displayed the first zeal, having cast aside their military belts, but afterwards returned, like dogs, to their own vomit ... "
By Nicea, Christians were forbidden to serve in the military — interesting, as many did serve, and suffered for their faith — just a few years before. I wonder what Constantine thought of this. Also that he, as overseeing sacrifices to the Roman gods, could not be admitted into the Church, which was perhaps why, despite his support for all things Christian, he was not baptized until he retired as emperor on his deathbed.
Canon 20:
"Forasmuch as there are certain persons who kneel on the Lord's Day and in the days of Pentecost, therefore, to the intent that all things may be uniformly observed everywhere (in every parish), it seems good to the holy Synod that prayer be made to God standing."
In
De Corona (ch. 3) Tertullian (c200AD) says that the practice of praying while standing on Sunday and between Passover and Pentecost was a long-standing tradition. So this tradition predates Nicea by around two centuries, at least.
To be clear, I am aware there was friction going back to the Bar Kochba revolt between Jews and Christians, but not sufficient to completely separate the two as began at Nicea and appears to have been mostly completed 40 years later at Laodicea.
I think the separation began in 70AD when Jewish converts were forbidden to attend the synagogue.
If you have any evidence of anti-semitism prevalent at Nicea, I'd be interested.