My wife and I will sign our Last Will and Testament in a few weeks. Previously, we had two wills, the earliest being 1989. At the moment we sign those wills, our wills of 1989 will have no effect whatsoever on the distribution of our estate. A judge will spend NO time considering that will, except for one reason: Intent.
If we had expressed an intent in the earlier will and then a rather different intent in the current will, a heir would challenge the last will on that matter and the court would be interested and resolving that issue.
So, when Christ died, a New Testament became active. The old Testament became null and void.
The Jews made all kinds of trouble for Paul about observing the Old Testament and Paul had a terrible time convincing the Jews/Jewish Christians that new (Gentile) Christians did not have to follow the laws of worship in the Old Testament.
So, the issue of observing the Sabbath Day is moot. From the earliest, Christians observed the first day of the week.
God’s purpose for the Old Testament was to little-by-little bring the people to be ready for Jesus Christ. In the KJV, the OT is referred as a “schoolmaster”, teaching little-by-little as time went by.
Having been in Egypt for 400 years, it is understandable that Israel was susceptible to idol worship, since idols were all over Egypt. Even Solomon was subverted by idols. But, on returning from Babylon/Persia to rebuild the temple, we hear no more about idols. Israel was closer to being ready for Christ and His New Testament.
Go re-read Romans 11.
Your first statement isn't accurate. There was no observation of Sunday as a day of worship OR as a replacement for the sabbath of Jesus Christ by Christians in the bible.
What you're looking at is taking some verses out of context and looking at them through a prism Sunday worship.
The biblical evidence is that the sabbath day of the Lord was observed. In the scenario that you're postulating the holiness of the sabbath day apparently disappeared without a hint of controversy. Remember, this is the same days that Christ CREATED as holy in Genesis chapter 2. It's the same day that Christ enshrined in the ten commandments. It's the same sabbath day that Jesus claimed he was the Lord of in three of the gospels. Yet nobody ever accused any biblical Christian of CHANGING the sabbath to a different day OR even attempting to. This would have been a MUCH bigger issue to the jews that opposed Paul then circumcision was and there is ample evidence that the circumcision issue was a huge controversy.