As for Sabbatarianism, we see Christian worship on the first day in Paul’s writing. and the earliest known Christian history. and thereafter.
Incorrect. A simple reading of traditional church history will show that the sabbath issue wasn't fully accepted until over 300 years after the death of Christ.
Canon 29 of the Council of Laodecia of the year 365 AD says this:
Canon 29
Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ.
So it wasn't apparent that Paul ever wrote about transferring the worship and solemnity of the Sabbath of Jesus Christ to any other day besides the one Christ created holy and kept holy.
In fact it took over 300 years for this to happen...and that was primarily because of anti-Jewish sentiment in the early traditional church.