From my view, witches seem to be entities basically that have a lot of roots in real world spirituality. In the concepts of humans ascending (or in Lambdadelta’s case as she explained it, descending) into their consciousness, or in simpler terms developing their spiritual senses and awareness, whether its through trauma (as for many of the witches, such as Beatrice, Bernkastel, Lambdadelta, Ange - even Maria) or through other means such as Featherine and Virgilia, and Eva Beatrice (swollen with pride and drunk on the occultic world of her Father, Kinzo) or in Battler’s case, a form of enlightenment
Like spiritual things, witches are connected to the psyche of their originator. Bernkastel for example, while a witch from outside of Rokkenjima, was born from Rika’s trauma. And its not that witches are necessarily coping mechanisms or personalities as an antifantasy sense, but an evolution of consciousness into a world beyond the physical, that the spirit of the person evolves into beyond their own physical self, if that makes sense.
The meta world reads a lot like the astral world to me, and witches a lot like a spiritual self, who exists in the astral/spiritual world but connected to the physical world (and can incarnate /interact with the physical world, kind of like what “pieces” are) or that a “piece”, like Battler, can ascend into this world and be conscious of it, like an astral traveler.
There’s more to it but I dont want this to get too long, but essentially witches, and the meta world, seemed very much like that to me. Especially since the meta world goes beyond Beatrice and her own magic, and there are outsiders with no real world counterparts. We know the Siestas and 7 Sisters, and Sakutaro, but not the SSVD or Eiserne Jungfrau. Bernkastel and Lambdadelta are also outsiders, and especially Featherine and all of her servants. As Arietta mentioned as well its confirmed Alice is another witch outside Umineko, so I believe the meta world and witches could be a metaphor for basically the spiritual world in the WTC universe. (And that being said - to a Japanese person the spiritual isnt a seperate, far off place from the physical, but rather the other side/inherently connected to the physical world - like two sides of the same coin) which would also fit into that concept of the meta world.