Well, if it helps, and I hope it will, the author of Revelations wrote Revelations before there was canonization into the Bible we know today. In other words, when Revelations was written it was a stand-alone “book”. The Bible didn’t exist at the time.
So, when the author of Revelations says ‘don’t change anything in this book’, they mean and are talking about the Book of Revelations—not the Bible as a whole which didn’t exist.
In terms of the ethical value of things like the Gospels, the value is still there. The teachings we have are the teachings we have, if they are valuable to you then that’s great and you should follow them! Jesus says a lot of things which are great to practice. But it’s ultimately up to you to decide whether to believe in the teachings, and then it is up to you to struggle to put them into practice. That doesn’t change.
If you found out there were some changes to the Bible, would you stop believing in “love your neighbor as yourself”? No, because it still has value as an ethical teaching. If you want it to be the direct word of God, then that might be more difficult to prove as fact. But I choose to believe that what Jesus says sounds like what I would expect from a God anyway so I don’t personally have an issue. I hope it helps you though, it is ultimately your decision to choose what, how, and why to believe in something.