Luke 24:27 we have "all the scriptures"
25 And then He said to them, “You foolish men and slow of heart to believe in all that the prophets have spoken! 26 Was it not necessary for the Christ to suffer these things and to come into His glory?” 27 Then beginning with Moses and with all the Prophets, He explained to them the things written about Himself in all the Scriptures."
So it is reasonable that the term as used in the NT refers to the OT canon of scripture known to Jews at the time of Christ as we see in Luke 24. And when you link it to vs 15 - the scriptures that Timothy was reading in childhood - it is pretty certain that this is the primary reference.
Other translations say "as they do with the REST of scriptures"
2 Tim 3:14 Therefore, beloved, since you look for these things, be diligent to be found spotless and blameless by Him, at peace, 15 and regard the patience of our Lord as salvation; just as also our beloved brother Paul, according to the wisdom given him, wrote to you, 16 as also in all his letters, speaking in them of these things, in which there are some things that are hard to understand, which the untaught and unstable distort, as they do also the rest of the Scriptures, to their own destruction. NASB
Which means Peter is including the writing of Paul in "the rest of scriptures" . Where the "rest of scriptures" includes the OT and other inspired writings of the NT age.
Acts 17:11 they "studied the scriptures daily to SEE IF those things spoken to them by the Apostle Paul - WERE SO" must be the OT text primarily until more NT letters would be added.
Paul himself makes the point that his words were being accepted as "The word of God".
1 Thess 2:13 For this reason we also constantly thank God that when you received the word of God which you heard from us, you accepted it not as the word of mere men, but as what it really is, the word of God, which also is at work in you who believe.
I don't think 2 Tim 3 makes the claim that only those things God has inspired in the past are scripture or are useful for doctrine. The text is not arguing against current revelation but is making the case for sola scriptura testing of all doctrine and tradition. It is not trying to create an "end point" for when scripture stopped being produced as inspired by God.
I agree that this is not a text about Paul attempting to marginalize one group of scriptures as compared to another or an attempt to diminish NT scripture.