You can make it a couple times better by doing a close approach to the Sun. Within Mercury's orbit, you'll get 16 times (apologies for my napkin-quality math) the push it'd get around here.
And, with some mirrors, you can make a solar-thermal rocket as well.
All that makes me think: if today we decided to launch a Voyager-mass probe, what would be the maximum speed we could achieve with technologies we could deploy in a couple years (ruling out NTRs, liquid reactors, nuclear lightbulb or anything that requires a non-RTG fission reactor on board).
Imagining a chemical rocket for the initial push, then solar sail (maybe solar-thermal-rocket as well) from the perihelion until the distance to the Sun makes it worth to jettison the sail (maybe reorient the mirrors towards solar panels) and make the rest of the delta-v on ion engines powered by the solar panels increasingly replaced by a set of dedicated RTGs.
In some orbits around Earth and other planets with a string magnetic field, you can drive a ship by pushing on the planet. Also pretty low propulsion.