Two ways of solving this: via Greek and via Latin.
Greek first. I don’t care if the Aeneid is in Latin.
- Iliad: Nominative Iliás, Genitive Iliádos, so the stem is Iliad-. (The nominative in proto-Greek would be *Iliad-s.) Hence, Iliad-ic.
- Odyssey: Nominative Odússeia, Genitive Odusseías, so the stem is Odussei-. First declension, –ikos didn’t attach to those, Latin does its own thing, mumble mumble stuff I don’t actually know, which leads to Odysse(i)-an.
- In Greek, the Aeneid is Aineiás, by analogy with Iliás. So if Aeneid were an originally Greek word, its adjective would be Aene(i)ad-ic.
- In Latin, for whatever reason, the Aeneid is Nominative Aenei-s, Genitive Aenei-dis (treating the word as Latin) or Aenei-dos (treating the word as Greek. So the stem is Aeneid-, not Aenead– in Latin (therefore Aeneid in English), and if we treat Aeneid as a Latin word, the adjective is Aeneid-ic.
Google: 239 hits for Aeneidic, 146 for Aeneadic, both of which look to be used by reputable sources.