My visualization for a billion looks just like my visualization for a million, and a year seems like a long time to start with, so I can't imagine a billion years.

Would it help to be more specific? Imagine a little cube of metal, 1mm wide. Imagine rolling it between your thumb and fingertip, bigger than a grain of sand, smaller than a peppercorn. Yes?

A one-litre bottle holds 1 million of those. (If your first thought was the packing ratio, your second thought should be to cut the corners off to make cuboctahedra.)

Now imagine a cubic metre. A typical desk has a height of around 0.75m, so if its top is a metre deep and 1.33 metres wide (quite a large desk), then there is 1 cubic metre of space between the desktop and the floor.

It takes 1 billion of those millimetre cubes to fill that volume.

Now find an Olympic-sized swimming pool and swim a few lengths in it. It takes 2.5 trillion of those cubes to fill it.

Fill it with fine sand of 0.1mm diameter, and you will have a few quadrillion grains.

A bigger problem I have with the original is where X says "It's really important to me what happens to the species a billion years from now." The species, a billion years from now? That sounds like a failure to comprehend just what a billion years is: the time that life has existed on Earth so far. I confidently predict that a billion years hence, not a single presently existing species, including us, will still exist in anything much like its present form, even imagining "business as usual" and leaving aside existential risks and singularities.