I can agree with this portion of your post, though not all of it.
The fact that all Scripture is inspired by God (2 Tim 3:16), on which we can surely agree, does not mean that only Scripture is inspired by God.
If your position is that an account of what happened to Mary at the end of her earthly life was left out of the Bible for a reason, again we can agree, though I assure you that I am not on this thread claiming to know the reason. :)
However, if you say that an account that Mary was assumed into heaven is not trustworthy because such an account is not in the Bible, this has the same logical status as saying that a list of 66 books in the Bible {Genesis, Exodus, Leviticus, ..., 3 John, Jude, Revelation} is not trustworthy because such a list is not in the Bible.
It's almost as though the early Church wanted the ex-pagan converts to feel at home and give them something familiar to focus on.
God gave us His word thru the scriptures, which can and does apply to all areas of our life. Other writings can inspire us or enlarge on what is written...but when it comes to that which is of a spiritual nature it is always best to adhere to what He has revealed in the scriptures.
As for Marys death...frankly if God thought it was important we know how she died...and anything other than she gave birth to Christ... I am certain He would have made that clear...otherwise it is simply not that important....
The Bible emphatically condemns the use of tradition as a source of authority because whenever tradition is set up alongside of Scripture, it eventually competes for the greater position and replaces it. It then is used to reinterpret Scripture.
2 Timothy 3:16-17 .......All Scripture is given by inspiration of God and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work.
Suffice to say....Complete means just that, nothing else is needed to be function and mature in ones relationship to God except a willing heart and taking the time to read and study.
We are warned ..... “not to go beyond what is written,”... 1 Cor.4:6.