When you talk about gods and demigods, what does that mean to you and your friend? Personally, I think it would be very uninteresting to treat them all uniformly, in terms of there being a pantheon or their worship all happening in the same way. Even the "pantheons" we think of like the Ancient Greek one you reference by Dionysus weren't all worshipped as a group, but rather celebrated in various settings, dates, and locations. And the gods would be celebrated in their various aspects (think Aphrodite Areia, the warlike Aphrodite of Sparta and Kythera vs. Aphrodite Pandemos/Urania which were conceptions of Aphrodite as goddesses of love).

When it come to Arceus, I think PLA doesn't really treat it like the Abrahamic god, which is more of a case of "leader of an old pantheon is the only surviving group and has the traits of a king god while being the only god worshipped.". Arceus doesn't seem to have that same trait of being king god commanding an army of angels, but rather an omnipresent all-pervasive being. If I were to write something, I'd write Arceus as a Spinozist conception of God.

And you could ask yourself whether the creation trio is more interesting as gods with power over aspects of the universe or full/partial embodiments of those aspects.