It is not a surprise that unbelievers will attack the Gospel According to John. It is similar to attacks on Genesis. Both John and Genesis are foundational books to Christianity so if you weaken or destroy the foundation you can topple the belief system.
It seems like the objectors are using inductive logic in trying to attack John. Let’s say John is the effect and so let’s look for what caused this effect. I agree that John is different in tone than the other Gospels. The objectors are saying that the cause of the difference, the effect, is a result of embellishment over time.
That is the objector’s theory. Does it hold up to scrutiny? It is hard to say without specific analysis. What has been embellished? We need specific objections to respond. I think it is unfair for any objector to ask you to defend their general objection. I would want them to say this specific verse is embellished or false or contradictory. Then I could respond and defend.
I think in all of the Gospels you can deduce that Jesus is God. Is the embellishment in John that John more forcefully portrays Jesus as God. Is that embellishment? Can you be more God than God? That makes no sense.
In opposition to the objectors I say the Holy Spirit guided St. John to write the Gospel just as the Holy Spirit guided every biblical author. No mere man said the things that are recorded that Jesus said in the Gospel of John. They are too deep and they are too profound and the sayings are consistent with the other Gospels and the entire Bible.
I love the Gospel of John more than any book of the Bible. It is so beautiful and powerful. It would be great to sit down with the objectors and read it together and have them point out the flaws and see if they hold. The reading of John might just cure them of their unbelief. The Holy Spirit can apply that Scripture to the objectors’ hearts as it is read. It would surely be just like God to bring an objector to saving faith while they attempt to attack His Word. He does things like that all of the time.