other ideas: 1. be way more patient 2. anti matter based propulsion (more out there than solar sails) 3. nuclear bomb based propulsion
One issue is as you get to these speed little bits of dust will anhillate the probe, so you need some kind of shielding, raising the mass budget, making it all the harder. A solar sail has to be able to survive holes getting poked it in it and still working, etc.