It is also the slowest spacecraft. Speed is relative. To get it going that 'fast' when close to the sun the other half of its orbit is correspondingly slower than earth. When launched it accelerated away from earth at speed, but from the perspective of the sun is was actually decelerating, and was moving much slower that it was when attached to the earth.

Great! , do that gravity assist a few more hundred times around the sun and we are there in 500 years.

You can't do a gravity assist around the sun once you've reached solar escape velocity, because you won't be visiting the sun again.

If we could pull that trick so easily, we already would have.

Well... you could wrap around another star, or two, and come back to this sun for another assist a la Rendezvous with Rama.