Oh, I entirely agree - compared to the exactitude with which the Torah was translated, and the whole of Tanakh, the treatment of the Septuagint (and the later New Testament) was egregious.
But, a few points:
Firstly, one cannot blithely take the whole of the Septuagint as a single body of work. It was created in stages. I am of the mind that the origination of the Septuagint, which was only Torah (the Pentateuch) was skillfully done - perhaps not with the precision of Temple scribes, as one must allow for the language barrier, but I think it was an honest effort... It is in the latter revisions that one might find question.
Secondly, your premise wholly ignores the politics of the time - Judaism was not pristine, and in a vacuum. The Hellenists, present even in Jerusalem, are a significant liberalization of Judaism - I think the Alexandrian codex, the Septuagint, was an honest attempt to teach Hellenistic Jews that which they purported to believe, Hebrew having been lost to them. It is the latter revisions which show that attempt being hijacked and Hellenized. Your opinion lacks for study into the construction of the Septuagint.
And the synagogues were another point of pressure - While Torah was pristine (IMO) across the board, what Torah supposedly said was widely disparate, and can be compared to Congregational Protestantism in our day (the basics were right, but not agreeing in complexity), and was just as highly factional, especially the further one got from the Temple... Temple Judaism was quickly losing ground to what later became Rabbinical Judaism.
It is for all these reasons that Protestant Christianity rejected the scriptures their fathers used and went directly to the source (as much as was possible).
But what will inevitably stand in your way (with regard to Messiah) will be the books of Daniel and Isaiah, both of which are present in the Dead Sea Scrolls, both of which are in the (proto)Masoretic tradition, and both of which, as extant-in-situ predate Yeshua by hundreds of years. Their accuracy cannot be ignored.
These guys were no different from a woman who eats ham sandwiches on Yom Kippur bringing out a kosher cookbook. No one who keeps kosher is going to touch it.
I get it - I can loosely be identified as a Messianic Christian. I try to keep Torah and kosher (Torah, not Rabbinical). But your premise assumes that any and every scribe not Temple ordained is incapable of an honest and literal translation. While I agree with you that bias and inclusions have crept in, that does not mean the truth is not there. As an example, IMO, for the lion's share, the KJV can be used with abandon - the milk of the message, and most of the meat, is there. It is only when one ventures in with specificity to very deep things that one must cross-check other variants to winnow out a meaning.
They even screwed up the first word. It does not say “in the beginning.” There is no suggestion of creation out of nothing. It says “at the beginning of....” and goes on to describe a process. It does not rule out pre-existing universes or other universes.
Yes, it most certainly does say 'in the beginning'. To say otherwise would offend the prophets and the Holy Days, and primarily, Shabbat.
There were no synagogues in Israel not until after the destruction of the Temple.
Porphyry, a Greek philosopher, using textual analysis exposed Daniel as bogus. That finding has stood the test of time. There is a litmus test on the Hebrew side as well. Angels do not give their names. He names two. The writer is a con artist.
The Greek translation of the Torah was a total botch done by numskulls out to make a fast buck and there was no need for it.
You don’t read Hebrew? Rashi pointed out the correct translation way back when “at the beginning of.....” a process. If you want to say “in the beginning” the expression is “barishona” not b’reshit.”
Anything else?