but it is said laser and solar sails can propel themselves from EM pressure
Ion thrusters accelerate slowly but they do gain in speed through time. In space one may assume the object will tend to remain in motion, and even small acceleration should add up over long periods of time. Also fission reactors need not be extraordinarily large, and shielding requirements depend upon the construction of the ship.
nuclear waste from which no more energy could be extracted could also be expelled to supplement efforts by reducing mass and providing additional propulsion