Space

You're not wrong about this thing being bombarded by tiny collisions, but we should note that the impacting bodies that made the craters visible in these images were a lot larger than dust or sand grains. It's not surprising to see an object like this, within the asteroid belt, covered in simple bowl-shaped craters. From what the New Horizons probe to Pluto and the Kuiper Belt has seen, however, even dust grains are actually pretty sparse in the outer solar system. Even if there actually is a "second Kuiper Belt" as some of the NH team proposes, it isn't that dusty. In interstellar space, I would expect dust grains to be even rarer, let alone sand or pebbles.

That being said, interstellar probes will definitely need some form of protection from hypervelocity impacts, however rare they may be. That's one good reason we should be looking seriously at more modest (and more feasible) proposals for spacecraft that will reach large but attainable distances beyond the heliosphere (say, 75 billion kilometres out from the Sun). The best way to verify the dustiness of nearby interstellar space is to measure it directly. Scouts have their value.