So what’s your opinion of Christian churches that say their focus is on the Old Testament?
I'm not aware of any Christian churches that "focus" on the Old Testament.
Messianic Jews if you consider them Christian, but they're already pretty controversial. Aside from them the closest would be probably Ethiopian Orthodox.
After those, I would say the next closest is probably someone like Steven Anderson who says we need to maintain the judicial components of the Law, e.g. executing people for bestiality. The idea that the ceremonial aspects of the Law were fulfilled by Christ is uncontroversial among Christians, for He is the true and eternal high priest, "a priest for ever after the order of Melchizedek." The sacrifice of animals was merely a sign pointing to the sacrifice of the Son of God.