I think it could be indeed that you have to rely on Chaonion Agema, but only when you can get it, and even then as a minority unit rather than a mainstay. Perhaps between the Phalangitai and Agema stages of development Epeirote players would have to rely on shock infantry and cavalry, the phalanx only being used to hold segments of the line. Hence the Illyrian units. No?
But I agree, it would certainly be nice to have a pezhetairoi-like intermediate unit for Epeiros available in its home provinces, something Makedonia could recruit too. It would be non-interchangeable with Pezhetairoi, and possibly slightly inferior, but certainly recruitable.
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