Regarding Coptic doctrines on afterlife or any other topic, it is important to note that Orthodox in general (be they Chalcedonian or Miaphysite) generally place great stock in "keeping the faith of the Apostles," even more than us Latins — who have a principle of doctrinal development. Thus, if you ask a Copt about their theology in Late Antiquity, they will tell you it is the exact same as their theology in the 21st Century.
That said, if you still subscribe to the position that some development must have taken place, then your best bet is to read the works of the Alexandrian Fathers. The standard work collating the extant documents of the Greek-speaking Fathers, Patrologia Graeca, has several writers who hail from Egypt; the most important ones are Clement of Alexandria (volumes 8 and 9), Origen (volumes 11–17), Athanasius (volumes 25–28), the Desert Fathers (volume 40) and Cyril of Alexandria (volumes 68–77).
Note that the Patrologia Graeca has all the writings then known from 1st to 15th Century theologians writing in Greek in any subject, so there is substantially more in there than simply views on afterlife; that the version composed by Migne has side-by-side versions in Latin and Greek (but not English), so we'd have to look elsewhere for translations of those in these languages; and that particularly in the case of Clement and Origen, part of their writings is considered heretical (generally having a Neo-Platonist slant). So tread with care.