What you’d probably have to do to open this up to general space travel in the solar system is to have some type of failsafe or deadmans switch in every manufactured craft that either automatically alters course from earth inside a large limit (like more than a million miles) or initiates a large self destruct device that essentially ionizes the craft if it passes the million mile limit over a certain speed.
Near lightspeed or even significantly fractional lightspeed projectiles would be fearsome weapons, indeed.
But you cannot (in the case we are talking about) get more energy out of a space ship than you pack into it.
I suppose that you could release all the power of a nuclear reactor, working for years, in fraction of a second, achieving, essentially, an atomic bomb.
I would be more worried, I think about somebody out in the asteroid belt lobbing rocks at earth. Rocks are cheap, and you already have quite a bit of energy from the Sun’s gravity.
If we start having star flight, we will have to post early warning systems around the system, I guess.
When you get above about .5 C, I do not see how even a warning system would work.