My understanding is that John is a less than reliable historical source, having been written well after the others and with a totally different style and conten

My understanding is that John is a less than reliable historical source, having been written well after the others and with a totally different style and content. There's a lot of emphasis on the divinity of Christ along with other theological points. That stands in contrast to other variants of Christianity at the time which rejected the divinity of Christ (gnosticism), or the physicality of Christ (doceticism). It's very possible that John was canonised as a way of selecting the most acceptable or spreadable variant, rather than as a true reflection of the teachings of Jesus. As you say there are allusions to "son of God" in the other gospels but it's far less frequent and more ambiguous, and may well have been interpreted/revised in light of the experience of Jesus' resurrection. I suppose bible scholars are splitting hairs, but as programmers we know that sometimes you have to split a few hairs to get the bottom of things.