If history is any guide, then the actual best media for very long-term storage of data is engraving it into stone or shells. We have examples of engravings that have survived for nearly half a million years, by far predating the arrival of modern humans and even older than the oldest Neanderthal bones that have been found.
The oldest known rewritable long-term storage media is just 8,000 years old, and that's probably only because we didn't manage to invent writing until around then, and thus created a need to fix typos. The clay media can be rewritten as many times as needed until it is finalized by firing.
So if you are trying to leave a message for the lizard people who will arise long after the demise of humanity, etching it out in large print onto something like glass blocks is the way to go. It is very unlikely they would even notice there was data in a sequence of pits on a shard of DVD or even 5D optical data storage (the memory crystals put into Musk's Tesla in space and Microsoft's 10,000-year Git depository archive) because they would have to intentionally go looking for it and it may never occur to them to do so.
BTW just this year, Superworms have been discovered that can survive on just polystyrene. Just like a cow cannot actually digest cellulose but uses symbiotic bacteria in its gut to do it for them, bacteria that can generate enzymes that break down plastics also live in the gut of those Superworms. Why is this important? All of the coal in the world was deposited between 360 million and 200 million years ago because that was the time between when plants invented woody lignin (which allowed for the first trees) and bacteria + fungi developed a way to break it down into compost. It was expected that plastics would last for a similar length of time before evolution would figure out a way to break that down for food, but here we are not even a century after the introduction of plastics.
So it's glass or stone blocks, or fired clay if you don't need something quite so permanent.