I wonder if Origen’s ‘Contra Celsum’ preserves anything of interest.
Perhaps Celsus had some views too.
Also, fairly early in Church History, there are biographies of Moses, such as St Gregory of Nyssa’s Life of Moses. There are also various Muslim traditions that, like the oral Church traditions of Christianity and the rabbinical traditions of Judaism, are extra-Quranic.
Even this may not exhaust the possibilites. I wonder whether the Mandaeans, or some of the other less well-known religions, have some interesting ideas. As well as the various heretical sects of Christianity that no longer exist, or long-gone forms of Judaism and Islam.