Just how fanciful is Egyptology?

You do need to be careful when studying Ancient Egypt, since there are some very ... questionable and ... selective ... interpretations available.

In addition, some work by earlier authors (for example Wallis Budge) has been superseded by more recent research. As Budge's Wikipedia page observes:

"... since his day both translation and dating accuracy have improved, leading to significant revisions. The common writing style of his era — a lack of clear distinction between opinion and incontrovertible fact — is no longer acceptable in scholarly works."

As with all historical studies, our understanding of the past can change enormously based on chance discoveries of new information.

Unfortunately, many of these outdated texts are now out of copyright, and cheap modern editions are available which may create the impression that the information contained in those editions is also up-to-date. Furthermore, as the quoted passage from Wikipedia above makes clear, those texts also sometimes presented scholarly opinion as fact. That combination can be problematic.

Reputable Egyptologists today are generally very clear about the limits of our knowledge. If we don't know something, they will say so. If something is an interpretation based on incomplete sources, then they will also make that clear.

However, sometimes that nuance can be lost in popular texts on the subject. The Ogdoad is a case in point (see below).

For the most part, our evidence for Ancient Egyptian beliefs comes directly from the ancient texts and inscriptions discovered in Egypt. These sources may be supplemented by foreign writers who described events and practices in Egypt (often not particularly accurately!).

For a good introductory overview of Ancient Egyptian mythology and beliefs, you could do a lot worse than Geraldine Pinch's Handbook of Egyptian Mythology (Oxford, 2002). She includes references for further reading and details of primary sources for each entry.

For your example of the Ogdoad, we have a large body of evidence dating back to the Old Kingdom, but those sources do not always agree on the details.

For example, I have seen popular texts and articles that confidently list the names and attributes of the eight gods who make up the Ogdoad. However, the truth is that even the identities of those gods are not consistently recorded in the original texts.

The deities who make up the Ogdoad differ from one source to another. Nun and his female counterpart Naunet, the deities of the primeval waters, are nearly always included. Naunet may be a primeval form of the sky goddess, Nut. Amun and Amunet, deities of invisible power or the breath of life, are in some of the oldest lists. When Amun was regarded as a creator separate from the eight, he and Amunet were replaced by Nia and Niat, deities of the void. Primeval darkness was represented by Kek and Keket or occasionally Gereh and Gerehet. Some lists have Tenemet, “chaos,” or Heh and his female counterpart Hehet. Heh and Hehet are difficult to interpret. They may originally have embodied the strong currents in the Primeval Waters. The Ogdoad of Hermopolis was sometimes treated as identical with the group of four or eight Heh gods created by Shu to help him support the sky. They in turn were sometimes identified with the “Eastern Souls,” the eight baboons who helped the sun to rise.

• Geraldine Pinch, Handbook of Egyptian Mythology, p176

That was written by an Egyptologist, and as you can see, the limits of what we know from the sources are explicit (Geraldine Harris Pinch is an Egyptologist with the Faculty of Oriental Studies at Oxford University).

My advice would always be, when in doubt look at the sources cited by the author. If the author does not cite their sources, I'd look for a different author!

For her entry on the Ogdoad (quoted above), Geraldine Pinch has the following:

References and further reading:

• S. Tower Hollis. “Otiose Deities and the Ancient Egyptian Pantheon.” Journal of the American Research Center in Egypt 35 (1998): 61–72
• L. H. Lesko. “Ancient Egyptian Cosmogonies and Cosmology.” In Religion in -Ancient Egypt, edited by Byron E. Shafer. Ithaca, NY, and London: 1991, 88–122.

Primary sources:

• PT 301; CT 76, 78–80; Leiden hymns; MT; Khonsu Cosmogony; BOF

Those primary sources are listed in her appendix.

• PT = Pyramid Texts

• CT = Coffin Texts

• Leiden hymns = See Foster, Hymns, Prayers, and Songs, 68–79.

• MT = Memphite Theology

• Khonsu Cosmogony = See R. A. Parker and L. H. Lesko. “The Khonsu Cosmogony.” In Pyramid Studies and Other Essays Presented to I. E. S. Edwards, edited by J. Baines. London: 1988, 168–175.

• BOF = Book of the Fayum

Which allows students of the subject to take their research further.