I suppose it can’t build up enough speed by going in direct line out of the solar system as the falloff would mean there’s not enough “runway” so to speak. But I wonder if you could circle the sun and “tack” like a ship does sailing into the wind and build up more momentum that way?
You could have an eccentric orbit that swings far out into the solar system and then when it approaches the sun again accelerates to reach the escape velocity of the sun. But that would take years.
And I think still only be rather slow once it escapes the solar system since the escape velocity would be almost used up.
There is the concept of a nuclear photonic rocket - Wikipedia which I think of as a light sail and a white hot glowing lump of nuclear fuel on a string.
