As we've discussed before, you and I, I think the "fuzzy" enters, in part, from a lack of work and introspection in the Scriptures on this topic of Law as well as from a natural and, unfortunately, widely taught bias against written Law.
And, yes, as you say, "keeping only some of the law of Moses." At the current time, I'm one of those who thinks the Law of Moses as a unit was ended with the Mosaic Covenant and era, and a change in Law was made (Hebrews), not only to facilitate the Great High Priesthood of Jesus Christ in this NC era, but also to end animal sacrifice and the Aaronic Priesthood and certain Laws that may have had to do with separation of Jews and Gentiles (although Gentile proselytes could join Israel and Israel was to be a Priest Nation to the nations). But, on the other hand, any Commandment that can be summarized under the Love Neighbor commandment of Torah, and any other commandments (besides the Love for Neighbor commandments) that are still active that can be summarized under Love for God commandments from Torah, are all part of the changed Law in Christ. The fulfillment of Messianic Law requires being
knowledgeable of all sin/lawlessness and assisting one another if/when caught in any sin/lawlessness to be restored to order (
Gal6:1-2). The bulk of lawlessness is stated and taught in OC written Law.
From there, as we've discussed, you and I, it's all a matter of what wisdom about and of God can be gleaned from studying all His commandments. For example, how the animal sacrifices taught about the final Sacrifice, etc.