i can read entire paragraphs in the Septuagint, it's written in an easy style and i enjoy being
able to read chunks of it. But i wonder if mixing Koine and Attic may do more harm than good. Some input would be appreciated.
No way. Any Greek you read will help your Greek. The Septuagint probably is the easiest Greek ever written because as translation Greek it is based on a Hebrew Syntax which is very simple, and rather similar to English. The Septuagint, particularly certain books like Amos and Ecclesiastes, can be very frustrating because sometimes the translationese is so bad the it produces unintelligible Greek. In some places the Hebrew Vorlage was probably corrupt. But if you stick to certain Books like Kings and the Pentateuch, you will be okay. Many of the words found in the LXX are classical and even Homeric. So, keep on reading and make no apologies.
οὐ μανθάνω γράφειν, ἀλλὰ γράφω τοῦ μαθεῖν.