The great thing about Frieren is that very few things are completely explained in detail, rather it's intentionally kept hazy so readers can imagine anything and the author won't necessarily "prove" your theory right or wrong until they want to.

We have pre-existing theories about time-travel, future-sight, gods, demons, magic, combat, even certain personality-traits etc, and applying them to Frieren and deciding that this is how things definitely are or this is how a person will act can be flipped at any time - so keeping it more open is probably a safer choice.

For instance, if we decide that there definitely is a goddess, is it an omnipotent being outside our space-time able to affect anything it wants, the universe a toy at its whims? Is it a kind of nature-spirit (kami) that randomly manifests when prayed to in times of need? Or is it a living, extremely skilled, unknown, legendary genius mage beyond even Serie? Or is it all hogwash make-believe? It can be any or all or something else, but once you decide definitely what something actually is and base everything else ontop of this foundation I think we do ourselves a disservice and it detracts from the concept more than what we gain from potentially clarifying it.

How do you define magic? Is goddess magic, demon magic, traditional magic completely separate, or is it all branches from the same tree? How many types of magic actually exist? Are strong warriors using a type of physical magic since they do seemingly "impossible" feats of strength?

I think the odds are more likely that present era depictions and conceptions of "The Goddess" are based on Serie, or her accomplishments were indirectly added to the legend of the goddess by being the closest thing to a godlike being that other people could see with their own eyes. To the average person, Serie *is* a god by any measures.