Existence is an indivisible state, by definition. The mystery of existence, at least until it is resolved, is a similarly universal condition. We can, however, delve into that mystery from a multitude of perspectives.
We cannot delve into a multitude of perspectives when we have defined the Mystery as Being-as-One.
I see the concept of Existence as indivisible Being-as-one is fundamentally sterile - I mean "sterile" here as the opposite of "fertile" or "productive", because we cannot derive anything logically from an atomic and featureless concept of Being-as-One.
"Existence exists" is a tautology that tells us nothing and cannot open up any further inquiries, because as you correctly argued, this concept of Being is indivisible and therefore cannot yield the vast diversity and multitude that are humanity, human society, science, nature, the universe, and so on.
In short, my argument here is that, when we contain the Mystery in this logical concept, we close all forms of inquiry and render it philosophically and existentially fruitless, we reduce a potential multitude of meaning to a singularity. Existence can be seen as divisible, and in fact infinitely divisible, if we do not let ourselves be shackled to this form of logical reduction.