If you can seed enough intelligence with self replicating "nano-machines", which seems reasonable, then you can accelerate clouds of that material at quite clos

If you can seed enough intelligence with self replicating "nano-machines", which seems reasonable, then you can accelerate clouds of that material at quite close to the speed of light with high power lasers, much like diffuse solar sails. No need to carry large payloads, reaction mass, or energy... those are at the endpoint where you find them (and convert them into grey-goo?). It's more of a viral mode of expansion where (virus) seeds carry the operational commands to new worlds (cells) which host them until they are consumed by the continued expansion. The intelligence and power consumption are concentrated where it's available, while passive catalytic action and stored knowledge is transported at speed.