It does not say in Hebrews 8:6-10 that the fault that God found with the Mosaic Covenant was with His law, but with the people for not continuing to obey it, which is why the New Covenant involves God putting His law in our minds and writing it on our heart so that we will obey it. In Psalms 119:29-30, he wanted to put false ways far from him, for God to be gracious to him by teaching him to obey the Mosaic Law, and he chose the way of faith by setting it before him, so this has always been the one and only way of salvation by grace through faith. In Exodus 33:13, Moses wanted God to be gracious to him by teaching him to walk in His way that he and Israel might know Him, in 1 Kings 2:1-3, God taught how to walk in His way through the Mosaic Law, and in John 17:3, eternal life is knowing God and Jesus, which is again salvation by grace through faith. The Mosaic Law is God's instructions for how to believe in Him, so we can't believe in Him instead of obeying His instructions for how to do that. Those under the New Covenant don't need to be taught to know God because it involves God's instructions for how to know Him being put in our minds and written on our hearts, so the problem is that many
people want nothing to do with the New Covenant.
The Mosaic Law is God's word and Jesus is God's word made flesh, so it is contradictory to believe in Jesus instead of obeying the Mosaic Law. The Son is the radiance of God's glory and the exact image of His nature (Hebrews 1:3), so Jesus embodied God's nature by setting a sinless example of walking in obedience to the Mosaic Law and the way to believe in him is by following his example. For instance, by doing 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 also expressing the belief that God is good, or in other words, we are believing in Him. Likewise, doing what is holy, righteous, just, merciful, faithful, and so forth in accordance with Christ's example is the way to believe that he is those traits, or in other words, it is the way to believe in him. This is why there are many verses that connect our faith in God with our obedience to His commands, such as Revelation 14:12, where those who kept faith in Jesus are the same as those who kept God's commands.
Our salvation is from sin (Matthew 1:21) and it is by the Mosaic Law that we have knowledge of what sin is (Romans 3:20), so while we do not earn our salvation as a wage as the result of having first obeyed it, living in obedience to it through faith in Jesus is nevertheless intrinsically part of the gift of him saving us from not living in obedience to it.
Either Paul only spoke against becoming circumcised for incorrect purposes or according to Galatians 5:2, Paul caused Christ to be of no value to Timothy when he had him circumcised right after the Jerusalem Council (Acts 16:3) and Christ is of no value to roughly 80% of them men in the US.
The Jerusalem Council did not rule that Gentiles were not under the Mosaic Law. In Acts 15:11, it makes it clear that Acts 15:10 was not referring to the Mosaic Law as being a heavy burden that no one could bear, but rather the heavy burden was an alternative to salvation by grace, namely salvation by circumcision that was proposed in Acts 15:1. The purpose for which God commanded circumcision was not in order to become saved, so the Jerusalem Council upheld the Mosaic Law by correctly ruling against requiring circumcision for an incorrect purpose. The Jerusalem Council did not have the authority to countermand God, so they should not be interpreted as ruling against Gentiles obeying the Mosaic Law, which would be ruling that Gentles shouldn't believe in Christ or have the gift of salvation.