If time was a reality higher than the ultimate reality, what you are calling the ultimate reality would not be the ultimate reality.
The point is that we don't know and we may not be able to know if the fundamental nature of space-time is or is not also a fundamental aspect of noumenon (a.k.a. Ultimate Reality/The Truth/[G]god/The Source) (mysterium invisus/magnum mysterium).
For time to exist, there must be existence. Existence precedes time. Without existence, there can be no time.
For the human concept and experience of space-time to exist, there must be humans.
We cannot say for certain if "existence precedes time" without knowing fully and exactly what "existence" and "space-time" actually are fundamentally and where they "came from".
So to be clear, I dispute that this is an observational issue. Obviously, you can't observe anything outside of time, and being time bound creatures it would be impossible for us to observe eternity directly.
Exactly. And as such, it is also impossible for us to determine if "eternity" even "exists" at all.
The Ultimate Reality is certainly eternal and unchanging. Otherwise, it wouldn't be what it is.
The bald assertion of, "The Ultimate Reality is certainly eternal and unchanging" is not logically sound.
When you say, "otherwise, it wouldn't be what it is" you are simply taking a stab at an ad-hoc ontological argument.
There is no way anyone can tell if noumenon (a.k.a. Ultimate Reality/The Truth/[G]god/The Source) (mysterium invisus/magnum mysterium) is either "eternal" or not or "unchanging" or not because it is unobservable either directly or indirectly and defined as the unknown/unknowable.
In the same way that being able to use a computer (observable phenomenon of space-time) doesn't mean you know how to make a computer from scratch (understand the fundamental nature of space-time).
If you don't know (or can't know) the fundamental nature of "time", you have no way of saying "before time - everything was eternal" or "without time - everything was eternal" you can't simply assume that what may or may not be beyond our epistemological limits either is or isn't subject to something like or some aspect of "time".
What you don't know is not automatically the opposite of what you do know. [LINK]
You can't really say what it is.
You can't really say what it isn't.