I agree that there are some passages in the scriptures that indicate that the old covenant was based just on the ten commandments. But I think there's other passages that indicate that it's based on many more commandments.
I attribute the difference to books like Exodus probably being the writings of more than one author, possibly compiled into its present form during the Babylonian captivity.
I agree that the vast majority of Christians observe God's rest on every day of the week.
How about if I reword it this way,
"Would you agree that the majority of Christians do not refrain from physical labor such as mowing the lawn or running the sweeper/hoovering on Saturday?"
Certainly there's going to be a lot of overlap between the 10 commandments and what is written on our hearts.
But there will also be some differences, imo.
Most Christians agree that being proud is not God's way. But that's not one of the 10.
Most agree that God wants us to hold the Bible in high regard.
Meet together frequently with other believers.
Again, not in the 10.
So I think what God writes on our heart is ideas, approaches, a lifestyle. There's going to be a lot of overlap with the 10 commandments and other laws given through Moses, but I don't think God writes a set of words, He gives a spirit.