The idea that the earth is only 6,000 years old is not a WTBTS doctrine or teaching.
I was, however, persuaded to entertain the other ideas you mentioned because I was persuaded to believe that JWs had "the Truth." And when I say, "entertain," I mean that I tentatively accepted that they MIGHT be true because I accepted that the religion was true.
I have since come to know that these are demonstrably false premises. No one individual or group can possess "the Truth" to the exclusion of others. Truth is not a tangible, concrete item which can only be in the physical possession of a single entity at a time. That is just ridiculous. Also, even if a group does have SOME ideas right, it does not follow that they have ALL ideas and teachings correct.
My reason for tentatively accepting the various JW doctrines was that, for a time, I really believed that Jehovah's Witnesses taught truth and were God's visible organization on Earth. Once I saw the WT leaders and the entire religion for the scam that it is, there was no longer any reason for me to believe tenuous doctrines that never had any independent basis in the first place.
In other words: I accepted JW beliefs because I was duped into believing JWs were special. It was NOT the case that I became a JW because I a priori believed their unique and often bizarre beliefs and unusual interpretations of scripture.
It all flows from buying into the idea that there is only "one true religion." There is not. That is a fallacy.
Let's review: It's a cult!