Your definition of “Truth” as only what is measurable within “existential reality” assumes a metaphysical position while claiming to avoid one. To say that only the measurable exists is not a scientific conclusion—it is a philosophical stance, one that excludes entire domains of thought, theory, and even aspects of physics that deal in probabilities, indirect observation, or emergent properties.
Even in physics, what is “measurable” often depends on model-dependent realism, theoretical constructs, and instrumental interpretation. If truth is bound strictly to direct measurement or forensic account, then foundational aspects of modern science—like quantum states before observation or cosmological inflation—would also fall outside “Truth.”
You draw a hard line between reality and approximation, but approximation is intrinsic to how we access even the most "empirical" data. All observation is filtered—through instruments, through models, through minds.
To say “nothing exists which does not exist in existential reality” may feel precise, but in practice, it's circular: it simply redefines existence as what fits your framework. That may be internally consistent, but it’s hardly the final word on Truth. It limits Truth to one method of access, and in doing so, quietly excludes the very richness of reality that inspires both science and philosophy.
Good day, and thank you for the thoughtful engagement.