By your own admission, God has not preserved his pure words for us to have and study. Did not God promise to preserve His holy, pure words for all generations? Where are they?
You say we have nothing perfect, what you have and what the KJV has is full of errors. What would a lost world think about that? We have no defense concerning truth, if we don't have truth. How are we to put our trust in the Savior of the Scriptures when we can't trust the Scriptures?
You are still quoting that one verse from Psalm about word preserverd for all generations. OK.
Did David writing the Psalm have all KJV books? No. He had the Law and nothing else. Maybe some historical books like Judges, not sure.
Did the generation after David have all the KJV books? No.
Did the generation during prophets like Isaiah or Nehemiah have all the KJV books? No.
Etc, etc.
So my position about this is, that every generation has the word it needs. We dont have any manuscript or NT copy without any error, so apparently we dont need it.
We have very good and well preserved Word, suitable for our needs. Every compilation of manuscripts (TR, Nestlé Aland, Byzantine, LXX...) are the work of (godly) men, so it will always have some minor errors here or there.
That does not cancel the Bible as such, we know in 99% of NT what was the original reading.
Its worse with OT, but we are not under OT anymore, so we can live with that.
KJV is a very good translation of NT from good source (TR), very good translation of OT, but from not very good source (MSS).
LXX and majority text of NT are for me the best available texts, but they are compilation of various different copies, we must know that.
This is my position and opinion, just responding your question, not saying you must read LXX. Stick to your KJV, its a good Bible. But if you will tell me its without any error, I will stand against it and show you many.