Was that it could only achieve a low fraction of lightspeed before the speed & energy of incoming protons outstripped the magnetic field strength of the scoop. It then had the same problem as before, pushing against debris, unable to go faster.

This is why I suggest clearing a path though space might be a better idea. In my idea interstellar debris isn't physically pushed aside, rather it is turned into something that lacks substance via weak nuclear interactions, so that the starship can go proceed forward unimpeded.

See? And I still have no idea if it would work. We know that neutrinos rarely interact with matter, they can pass through a lightyear of lead without colliding with anything. They are produced by Beta decay and nuclear fusion reactions. We don't know much more than that. In fact the seem to be less abundant than theory predicts. But suppose most of the dark matter in the universe consists of neutrinos, there must be a pretty effective way of making them! Well, that's grasping at straws, but I'm still thinking....