I mean I would just load couple of metric tons of antimatter in the ship and then just use up picked up interstellar space mass (1 atom per cubic meter is not m

I mean I would just load couple of metric tons of antimatter in the ship and then just use up picked up interstellar space mass (1 atom per cubic meter is not much, but you will be moving trough shitload of them) along the way to annihilate. the faster you move the faster you will go up to a couple of limits. BUT deceleration may be a bit of a problem.