You are conflating reference frames - it determines causality in your local reference frame but not in any other reference frame. The concept of "real time" - like all time - is entirely relative to the reference frame you are using.

Correct, there's no global timeline, so it's not clear what "just" 300M years ago even means. Some RFs seen it already, and some will never see it. But in ours it happened in Jun 2024. It's 300M light years away, not ago.

But there is a global timeline. The age of the universe itself. It happened when the universe was roughly 300M years younger. Somebody might say the universe was created a year ago if they traveled through it extremely close to the speed of light. But we know how fast we travel by measuring redshift/blueshift of the cosmic microwave background and it's definitely far from any relativistic speed. There must be some effect of gravity, but that is also within a rounding error. So, I'd say we are much closer to the "truth" than somebody who travels through the universe a few percent of the speed of light or "lives" right next to the supermassive black hole. Whether something we will never see exists or happens is a philosophical question akin to "if a tree falls in the forest..."

But a distant observer could also make the same measurement of the CMW background and see themselves at rest. And yet we could be moving away from either other at relativistic velocities, due to the expansion of the universe.

Due to this relative motion we will see different ordering of events at different locations, despite both being at rest relative to the CMW background, so we can’t really say there is a global timeline.

The thing is that the CMW background isn’t global, we don’t see the same background as a distant observer. CMW photons that are passing us now will take time to reach a distant observer, and vice versa.

There is a global timeline. It is distorted, just like space.

You’ve just blown my mind.

Another mind-blowing perspective is from that of a single photon; since time slows when approaching c, from its perspective it's eternal, and, since distance shrinks when approaching c, from its perspective it's also omnipresent.

Well, it's not exactly eternal, it's that time is not passing for that photon. It's also (as far as I can tell) only correct to say that distances shrink as speed increases when you're talking about massive objects. A photon would not experience space contraction in any meaningful way.

By eternal you mean the photon will "experience" being a phonon when it's absorbed by matter?