Frame-dependence is potentially fine I think? I don’t think “anything frame-dependent doesn’t have inherent existence, and our experiences clearly do” is correct, or at least this is dependent on how these words are being used. Certainly our experiences don’t have inherent existence in the sense of being causally isolated from the world and truly self-supporting.

I expect that what you are getting at here is the phenomenology of reflecting on “pure experience” or “the redness of red” and similar such things. When doing this, the sense that I get is like, yes, of course red exists in a sorta-platonic way that must be instantiated in literal physics. I am physics, red is observed, therefore we gotta have actual red in actual existence. Even if this is dependent on my particular way of perceiving, that way of perceiving is instantiated, so even if “red itself” doesn’t make sense, we can still get my experience of red by just reproducing the whole thing.

Anyway, agree with that stuff, but I don’t think this implies that our experiences are defined strictly in terms of frame-invariant structures in physical fields. The reason for this is because the substrate couples to the field, and is the only way we get to talk and think and observe the field anyway! Like, say partitioning of experience is dependent on the difference in field potential locally being high enough to get some substrate response to encode that structure in the field. Still frame-invariant in the sense that outside observers can agree on what experience is happening there, you just have to adjust to the local rest frame of the substrate.

A somewhat related point is that I don’t think we have the evidence necessary to say that the experiences need to be reference frame invariant like this. Like, I don’t think I can probe my experience from different relativistic frames in order to tell! The sense of frame-invariance is only up to our ability to actually check the axes of variation. The observation of independent existence and frame-invariance of experience doesn’t imply frame invariance for axes that you haven’t actually checked.

So yeah, I think the relevant causality and observations can happen without needing frame invariance of this sort, and I don’t think we can know at this point that we need frame invariance of the sort you mention. Our observational tools are anchored to our rest frames.