hi doma,
the old religions do have similarities, almost as if there was a prototype religion, just like there seems to have been a prototype language for the indo-european peoples.
it looks to me as if the original indo-european religion was highly sensitive to the seasons (agriculture/ movement of flocks), understood the calendar well enough to attempt definitions of solstices and equinoxes, associated particular gods with celestial bodies, assigned gods to specialised roles, expected arguments between deities etc.
so perhaps ishtar is everyone you think she is - and many avatars besides.
i also think that once the different indo-european peoples split in many directions, they kept developing their ideas. so these religions added and subtracted features in accordance with their circumstances. are the gods of asgard somehow related to the gods of olympus? is zeus another face of indra?
we are lucky enough to have some of the sacred texts of these ancient religions. and historians can detect common features in them. sometimes they can even trace a tree of life through them. so the origin stories in genesis have features in common with pre-existing babylonian mythology. and babylonian religion hearkens back to tales told in the clay of sumerian cuneiform. thus enkidu begot adam.
and if this is so, can we leap further back in time? can we get just a glimpse of memories handed down from a time before writing, before man had settled upon agriculture? are there resonances from gobekli tepe, from the paradise of the hunter-gatherer, to be found in the epic of gilgamesh?
"Then he, Enkidu, offspring of the mountains,
who eats grasses with the gazelles,
came to drink at the watering hole with the animals,
with the wild beasts he slaked his thirst with water."
gobekli tepe. with stone carvings of every kind of game and surrounded by hillsides covered with wheat. man in his eden. before the fall. from hunting to farming.
but i am not as certain as you about the derivation of the egg. it seems to pop up all over the place. it could simply be an obvious symbol of renewal and fertility. are eggs always associated with the moon?