>> “Welcome to the world of legalism.” <<
Obedience to YHVH is not legalism.
Legalism is the belief that you can save yourself through works and the adherance to the law.
Yeshua commanded the we keep the sabbath, and he did so himself, as did all of his disciples and his apostles. Every convocation written of in the NT was a keeping of a sabbath.
The Mosaic sabbath is the ceremonial sign for which the destination/goal is the rest one receives in Jesus.
As you know:
He made a point, several times, to violate the pharisaical understanding of the commandment to show the new covenant was near.
It was simply not carried over to the new testament, unlike the other, moral based commandments.
Paul, in three letters, writes to the effect of the futility of keeping the sabbath law.
As to Sunday:
Jesus, being resurrected, met with the faithful on that Sunday, pentacost comes on Sunday, Acts 20:7 shows a meeting on the first day of the week, breaking bread.