I think that Bluegoat makes a good point about canon being a somewhat minor issue between us.
However, I think that the reason it is brought up is this. The Protestant reformation is built largely upon the concept of Sola Scriptura - That the Bible is complete and inerrant. Yet at the time of the reformation the approved canon consisted of at least 73 books. Certainly this was the canon that Luther had.
So - seemingly - in one breath the reformers declared the Bible “complete and inerrant” and in the next breath they changed the “complete and inerrant” bible by removing books from it…

To me this was highly disengenuous on their part.
The fact that some of the books were questioned by various individuals over the centuries really has nothing to do the fact that Bishops of the unified Christian Church, and in multiple councils determined that the 73 books were canonical.
In large part I agree with this, howver we do have the passage in 2 Maccabees that is VERY strong evidence for Purgatory and praying for the dead. Protestants reject this of course as “not biblical evidence” - but who’s fault is that??
This is one that I will leave for the experts.
The one thing that I believe points to the larger canon AND coincidentally is based on an oft-used quote by protestants in support of SS is from St Paul who says, “ALL Scripture is God breathed…” and since, as you say above, Paul was in the Diaspora, writing to Timothy, a Bishop in the Diaspora, It seems very likely that Paul was refering to the OT used in the Diaspora - which would have been the LXX…
Peace
James