What does “thoroughly” mean to you? That it is complete. Nothing can be added to make it MORE thoroughly. If Scripture can make “the man of God perfect, THOROUGHLY furnished unto all good works”, what more can be added to make the man of God MORE THOROUGHLY FURNISHED?
Sufficiency, which is what you are referring to, merely states that nothing more can be added to scripture to make it more complete.
It does not say that only scripture is profitable.
What you are referring to is the indivisibility, that you can’t take books out of scripture which is sorta ironic given that you don’t really believe that all acripture is profitible.