One thing that will help in your discussion with Jonathan is that, as I recall from my prior discussions with him, ultimately he is not challenging the idea that faithful Christians are awarded with immortality.
He is challenging the idea that God created souls with immortality. Depending on how you paraphrase him, @Jonathan_Burke might agree that God created human souls with the intention of being awarded immortality … but it is not the natural state of the human soul.
Something to keep in mind , but which I don’t believe Jonathan has much interest in, is that the Essenes seem to parallel Zoroastrian metaphysics more closely than anyone else in the New Testament.
In a book usually associated with the Essenes (The History of the Rechabites), souls depart their deceased body and travel to a paradise island, permanently separated from all mortals by a river (or ocean channel?).
There, the souls reside in blessed contentedness until either the End of Days, or until such time that the Lord sends angels to fetch a person he requires to be in his presence.
What would be interesting to map is how the “dominant view” in the New Testament, of “sleeping until the resurrection”, was eventually displaced by the rival view that souls depart their bodies and exist elsewhere - - vividly displayed in such sentiments as people dying and becoming guardian angels to their loved ones!
The common Western view runs much more closely to the Essene view of “relatively immediate separation of soul from the body” than to anything the Pharisees taught… or even anything the New Testament teaches - - but with a couple of exceptions:
The parable of the Rich Man in Hell (speaking to Abraham in Heaven) appears to be more Essene than Pharisaic.
And the parable of the woman who marries 7 brothers may also be more Essene than Pharisaic.
The most notorious presentation of a view of afterlife that differs from the Pharisees or the Sadducees comes from Josephus’ discussion of the Eleazar ben Ya’ir’s inspirational speech in favor of martyrdom (the night before the final Roman breakthrough at Masada).
Whether this speech was actually made or not, Josephus is clearly telling his audience about a view of the afterlife that differs from the Pharisees and from the Sadducees.