google “kingdom of the cults”. It goes into JW in depth.
There are two doctrines that cause a group to be considered outside of Christianity. Those are “Who God is” and “The Plan of Salvation”. If you modify either of those two, you aren’t considered Christian.
JW have done both. They’ve denied the diety of Jesus. To do that they have their own translation of scripture that language historians agree, you can’t get that translation from the originals.
I’d focus on the scriptures that show Jesus was God.
Their salvation has elements of work in it. You have to do what they say to be saved. It’s not “saved by grace, not by works”.
No, they deny the trinity, that Jesus and God are the same person.