Jesus set a sinless example of how to obey the Mosaic Law, including keeping the 7th day holy, so he was much more religious and zealous for keeping it than the Pharisees were. He never criticized the Pharisees for obeying the Mosaic Law, but he did criticize them for not obeying it or for not obeying it correctly, so his purpose in criticizing them was not to get them to stop obeying what God commanded them to do, but in order to call them to a fuller obedience to it. For example, in Matthew 23:23, Jesus said that tithing was something that they ought to be doing while not neglecting weightier matters of the law of justice, mercy, and faith, and in Mark 7:6-9, he criticized them for setting aside the commands of God in order to establish their own traditions. In other words, his problem with the Pharisees was not that they were too religious, but that they were not religious enough.
In Matthew 4:17-23, Jesus began his ministry with the Gospel message to repent for the Kingdom of God is at hand, and the Mosaic Law is how his audience knew what sin is, so the sum of everything that Jesus taught in the NT by word and by example was to obey the Mosaic Law, which means that the degree to which you obey the Mosaic Law is the degree to which you value what he taught in the NT. Jesus did not establish the New Covenant in order to undermine anything that he spent his ministry teaching by word or by example, but rather the New Covenant was given for a time when the Israelites would return to obedience to the Torah (Jeremiah 31:33). Changing the medium upon which God's law is written from stone to our hearts so that we will obey it does not change the content of what it instructs us to do. The Spirit also has the role of leading us to obey the Torah (Ezekiel 36:26-27). In Romans 8:4-7, those who walk in the Spirit are contrasted with those who have minds set on the flesh who refuse to submit to God's law.
The Mosaic Law is how the Israelites knew what sin is and how we know what sin is (1 John 3:4), and we are obligated to refrain from what God has revealed to be sin through His law regardless of whether or not someone considers themselves have been given God's gift of getting to be under it. You should consider being judged by the law to be far preferable than to perish apart from it. In Psalms 119:29, David wanted God to be gracious to him by teaching him to obey the Mosaic Law, so that is what it means to be under grace and mercy, while wanting to not be under it is rejecting God's grace and mercy.
All of the commandments that God has given were specifically chosen in order to teach us how to love Him, which is why there are many verses in both the OT and the NT that associate our love for God with our obedience to Him. For example, in John 14:23-24, Jesus said that anyone obey loves him will obey his teachings, anyone who does not love him will not obey his teachings, and his teachings were not his own, but that of the Father, so anyone who refuses to come under what the Father has taught does not love Jesus, especially when everything that the Father has taught was specifically taught for the purpose of teaching us how to love Jesus.
It is not being a legalist to think that followers of God should follow what God has
command, but rather it is a sin not to. There is no need to remove the letters of Galatians and Colossians from the Bible, but rather there is only the need not to interpret those letters as speaking against obeying God's word.