Premises:
1. The Sabbath was a part of the Mosaic Law and of the Mosaic Law only.
Sin is what is contrary to God's character, such as with righteousness being in accordance with God's character while unrighteousness is contrary to God's character, and sin is the transgression of the Mosaic Law because it was given to teach us how to be a doer of God's character. The way to be a doer of God's character is straightforwardly based on God's character, not on a particular covenant, though God's covenants have included instructions for how to be a doer of His character are part of their stipulations, and God's character is eternal, therefore any instructions that God has ever given for how to be a doer of His character are eternally valid. For example, God's righteousness is eternal (Psalms 119:142), therefore all of His righteous laws are eternal (Psalms 119:160). So while the command to keep the Sabbath holy was included as part of the Mosaic Law, the significance of the Mosaic Law is what it teaches us about God's eternal character. Anyone who wants to know God through being a doer of His character should obey the Mosaic Law.
2. The Law was only for Jews, since Moses till Christ.
Jesus did not come with the message to stop repenting because the Mosaic Law has ended now that he has come, but rather he came in Matthew 4:15-23 with the Gospel message to repent for the Kingdom of God is at hand, which was a light to the Gentiles, and the Mosaic Law was how his audience knew what sin is (Romans 3:20), so repenting from our disobedience to it is a central part of the Gospel of the Kingdom. Jesus did not come to curse us by causing us to be free to do what the Mosaic Law reveals to be wickedness, but rather spreading the Gospel of the Kingdom is in accordance him being sent in fulfillment of the promise to curse us by turning us from our wickedness (Acts 3:25-26). Jesus is God's word made flesh, so he is not the abolishment of God's word, but rather he the embodiment of the Mosaic Law expressed through setting a sinless example for us to follow of how to walk in obedience to it.
The way to believe in God is by being in His image through being a doer of His character traits. For example, by being a doer of good works in obedience to the Mosaic Law we are testifying about God's goodness, which is why our good works bring glory to Him (Matthew 5:16), and by testifying about God's goodness we are expressing the belief that God is good, or in other words, we are believing in Him, and the same is true for other aspects of God's character. The way to believe that God is just is by being a doer of justice, the way to believe that God is holy is by being a doer of His instructions for how to be holy as He is holy and so forth. This is why the Bible frequently connects our faith in God with our obedience to Him, such as in Revelation 14:12, those who kept faith in Jesus are the same as those who kept faith in Jesus.
It is contradictory for someone to consider themselves to be a believer in the God of Israel while not wanting to be a doer of His character in obedience to the Mosaic Law in accordance with the example that Christ set for us to follow. The position that the Mosaic Law is only for Jews is the position that Gentiles have no need of the Gospel of the Kingdom that Gentiles have no need to repent from doing what God has revealed to be sin, that Gentiles have no need of salvation from sin, that Gentiles have no need of grace, that Gentiles have no need of Jesus to have given himself to redeem us from all lawlessness, and that Gentiles have no need of the New Covenant.
3. Even the book of Genesis was a theological part of the Mosaic Law and is not literal (namely in its creation account).
Please clarify the relevance of this point.
If these premises are all true, we are not supposed to keep the Sabbath.
The Sabbath is holy to God and what is holy to God should not be profaned by man, so we would still be obligated to keep the Sabbath holy even if God had never commanded anyone to do that. In 1 Peter 1:16, we are told to be holy for God is holy, which is a quote from Leviticus where God was giving instructions for how to do that, which includes keeping His Sabbaths holy (Leviticus 19:2-3), so the only way that we shouldn't obey God's instructions for how to be holy as He is holy would be if God we to cease to be holy.