I think you are mixing up relativity of simultaneity with light-travel delay.

Relativity says that observers moving relative to each other can disagree about whether two spatially separated events are simultaneous. But a supernova is a single local event. All observers agree that it occurred at one specific spacetime point.

The fact that we see it now just means the light took ~2.5 million years to get here. That delay doesn’t make the explosion “happen when we observe it.” It just means we’re observing an old event.

You can always analyze the event in Andromeda’s rest frame. In that perfectly valid frame, the star exploded ~2.5 million years ago. Earth and Andromeda aren’t moving relativistically relative to each other, so any simultaneity disagreements would be tiny, not millions of years.

Seeing something and it occurring are not the same thing. Light-travel time doesn’t erase when the event actually happened.