None of this stuff can withstand a particle of dust hitting the sail.
Or for that matter, a cosmic ray. It’s not going to end up being that small because it’s going to need to generate a powerfull magnetic field or carry a large physical shield to absorb particle collisions.
20% of c is a velocity creating kinetic values approaching what particle accelerators generate.
Also, is anyone considering the ramifications of an object like that entering a planetary atmosphere that happens to have a civilization on it? It probably won’t affect the surface, but any observer is going to know something epic just happened in the atmosphere...
On that line, a while back was a piece on some propulsive technology to get things moving around the solar system at %s of c. I commented that any type of ship utilizing it is going to have to have like a tenth AU auto destruct mechanism on it because slamming 400 or 500 tons of space craft into earth at 5-10% c is going to be a Kt level event... You’re going to have to have a nuclear device on board every craft that will be enough to turn it to plasma if it crosses that distance boundary on an intercept course!