It's also instructive to read the Old Testament scriptures that inform his understanding of Christ's sacrifice.
The first is Psalm 40 - specifically the Septuagint (Greek) translation, which is a little different than what is in the NIV (comes from the Masoretic Hebrew text, which is a later version by about 800 years).
You can read it in the Septuagint here:
https://biblehub.com/sep/psalms/40.htmThe first part is a theme in the OT that is like a drumbeat - that the sacrifices and offering under the Law were not in and of themselves what pleased God. This is repeated over and over and over again in the Psalms and Prophets. What St Paul (or someone else, possibly Apollos) is doing here is showing that the Psalms are teaching us what the alternative is to those sacrifices, or really what they represent and foreshadowed. That's what 10:1 says, the Law is a shadow of the good things coming.
So to him, now we can see clearly what the prophetic Psalm means - a body is prepared for the Messiah to do the will of God, and
that is what pleases God. The author adds an interpretive gloss to the Psalm in 10:8 by adding the words "for sin". Verse 14 is the conclusion of this thought, and in this light we see clearly that the perfection is the completion of...what? The perfection implies reaching an end. Clearly the end isn't sanctification or being made holy, because he said perfected (past tense) those being sanctified (present tense). The perfection is that of sacrifice, of worship. Our sacrifice and worship, is complete in Christ's sacrifice, in one offering. And not only in sacrifice in a general way, but the offering of the very same body that was prepared for Him.
This is why St Paul says, our actual worship is to present our bodies as living sacrifices - but our living bodies are the body of Christ, so our perfected worship is to participate in this same once for all (ephapax) offering.
The next two quotes are both from Jeremiah 31, so now the author is linking the sacrifice of the Messiah for sin with the new covenant. So the final and perfect offering for sin has opened up to a new way of living and interacting with God, which again was foretold. And it's
not the old covenant, but a new one. A new redemption story - not out of Egypt, but out from under death. A new and living way (v20).
This is the basis of the whole understanding of our confidence (v19) assurance (v22), our hope (v23), the reason we prod one another to do good (v24), the reason we even assemble as a church (v25) AND the reason we endeavor to keep ourselves from sin (v26-31).
That's the context of "perfect" in this chapter.