Josephus tells us that the Sadducees had no belief in the immortality of the soul. And that tends to support a view that the Torah (which the Sadducees followed) is sufficiently ambiguous to allow both that belief and the beliefs of the Essenes and the Pharisees to all be permissible readings.
Wouldn't a fundamental problem be, and apologies if this is overly secular as a reading, that there's no real set belief in Judaism which is then transmitted into Christianity? If you're trying to hammer together one narrative from texts written over a period of 700 years, with oral history traditions adding a few more centuries to that, then you're inevitably going to get texts at odds with each other as there was no single unchanging truth being transmitted. Just a series of texts reflecting the beliefs of individual writers from various points within that time frame.
eg Saul went and spoke to Samuel's spirit. Samuel talked back. At one point Judaism may have had some concept of life after death which wasn't a result of Greek thought. Perhaps an Egyptian conception of soul/body? But that was clearly not something which the person who wrote that story down wanted to write about.
You get to the time of the Hellenistic apocrypha and you really start seeing the Greek influences in it. You can push that further on to things like Philo's writings a bit later on too.
Definitely agree that the early non-canonical christian writings I've read have some element which at least strongly implies an immortal soul of some type. Just not sure how far one can then push that onto some form of proto-orthodoxy in 1st century christianity. Broad church was broad.