I would actually disagree with your argument here. For different reasons both traditional and historico-critical scholars tend to date John in the 90s AD. (Traditionalists because that’s when Irenaeus seems to date it). I take the minority position of Carsten Theide, Robinson, Daniel Wallace and a few others that it was written sometime in the 60s AD (after the death of Peter and before the fall of Jerusalem). Regarding the expulsion of Christians from synagogues I’d just say that a quick look at history shows us that religious bodies issue repeated anathemas against the same things all the time, that the anathema banning Christians from synagogues might have been a local rather than universal one, and that there’s no reason to think the anathema described in John is the same one as the anathema issued at Jamnia.
The reasons why I think it was dated early are because 1) it fails to mention the fall of Jerusalem as an example of fulfilled prophecy, even though John loves to point out other examples of fulfilled prophecy (including the death of Peter, betrayal of Judas, etc.), 2) it talks about the city in the present tense, and 3) goes to some length to point out that John the Baptist wasn’t the messiah, which seems to argue for an mid- rather than late-first century date. As for the high Christology in John this is only really a problem if you beg the question and assume that Jesus was a purely human figure who later became divinized (which is something that not just orthodox Christians but also neo-gnostics and mythicists, for different reasons, would all dispute).