"That would be limited to the OT since the NT had not yet been completed; the gospel and letters of John and Revelation not even being written until they were all dead except John."
Simply because the 'completed' NT canon was not yet confirmed did not make the already written NT books not any less valid.
Have you not read...is it not written?
I Timothy 5:17,18 The elders who rule well are to be considered worthy of double honor, especially those who work hard at preaching & teaching.
18For the SCRIPTURE SAYS, “YOU SHALL NOT MUZZLE THE OX WHILE HE IS THRESHING,” AND “THE LABORER IS WORTHY OF HIS WAGES."
The first Scripture is quoted from Deut 25:4. The second Scripture is quoted from the gospel of Luke chapter 10 verse 7.
Paul wrote I Timothy 4 years earlier (63AD) than 2 Timothy, around 67AD, just before the time of his death. So the Gospel of Luke had to have been written BEFORE 63AD & circulated, for Paul to have quoted Luke 10:7 AS SCRIPTURE, with the same authority as the OT Scripture in Deut 25:4.
The Gospel of Luke was considered Scripture by Paul, an apostle AT THAT TIME, even though the completed NT canon was not yet finished. It didn't make the Gospel of Luke any less Scripture than Deuteronomy.
Also, Luke was Paul's traveling companion, so his gospel was not ONLY (sola) tradition but that tradition was confirmed by an apostle & that tradition was based on actual apostolic authority to confirm Luke WAS SCRIPTURE not by sola tradition.
The APOSTLE Peter also stated in 2 Peter 3:15,16 that all the epistles of Paul WERE SCRIPTURE, just as the rest of the recognized Scripture, at THAT TIME of Peter's writing was confirmed. This was just before Peter's death, also about 67/68AD and AFTER Paul's death.
2 Pet 3:14-17 Therefore, beloved, since you look for these things, be diligent to be found by Him in peace, spotless & blameless & regard the patience of our Lord
as salvation; just as also our beloved brother Paul, according to the wisdom given him, wrote to you,
16as also IN ALL HIS EPISTLES, speaking in them of these things, in which are some things hard to understand, which the untaught & unstable distort, AS THEY DO ALSO THE REST OF THE SCRIPTURES, to their own DESTRUCTION.
17You therefore, beloved, knowing this BEFOREHAND, be on your guard so that you are not carried away by the ERROR OF UNPRINCIPLED MEN & fall from your own steadfastness.
The PROPHET Daniel also did the same in his book of Scripture, quoting from the written Scripture of Jeremiah & referring to the sacred writings of Scripture that he possessed AT THAT TIME, even though the OT canon was not yet completed. It didn't make the prior OT books or Jeremiah an less than Scripture written after that.
Daniel 9:2 In the first year of his reign, I, Daniel, understood from the SACRED BOOKS, according to the word of the LORD to Jeremiah the prophet, that the desolation of Jerusalem would last seventy years.
Scripture was inspired (God breathed out) by God. It was first spoken, passed on & then written down, whether OT or NT books. The 10 commandments were first spoken by God, then God Himself had Moses write them down on tablets. These tablets were broken & God had Moses COPY DOWN again on tablets the 10 Commandments. So we have the original destroyed but an exact copy made, just as valid as the original.
And Sola Scriptura, handled in a humble, spiritual sense, according to the APOSTLE Paul?
I Cor 4:6 Now these things, brethren, I have figuratively transferred to myself & Apollos for your sakes, so that you may LEARN IN US NOT TO THINK (GO) BEYOND WHAT IS WRITTEN, SO THAT NONE OF YOU--may be PUFFED UP on behalf of one AGAINST the other.