How do we tackle the argument that the OT seems to believe that the Mosaic covenant is eternal?

If he had really broken the law, would he have been righteous? No, he would not. You may certainly disagree.

There is a legitimate distinction between moral law, ‘laws of love’, and all the rest – ceremonial, dietary and civil.

Loving God means keeping his commandments, and his commandments are not burdensome.
1 John 5:3

He who is having my commands, and is keeping them, that one it is who is loving me, and he who is loving me shall be loved by my Father, and I will love him, and will manifest myself to him.
John 14:21 YLT