Laser propulsion does use reaction mass. Mass-energy equivalence means that the emitted light has mass. Conservation means that the spaceship loses mass as it fires the laser. It’s basically a standard rocket with a ridiculously high specific impulse.
In an old comment throwaway_yy2Di made me notice that in spite it has a ridiculously high specific impulse, the momentum to power ratio is very small, it's only 1/c = 3.3E-6 N/kW.
I believe that’s an inherent tradeoff. The faster you make your reaction mass go, the more efficient your use of mass, but the less efficient your use of energy.
IIRC if you don't need to create the particles like in a ion thruster, it is better to accelerate them as much as possible.
The problem with photons is that you need energy to create them.
If you double your exhaust velocity, you double your thrust but quadruple your power requirements. If you have a fixed amount of power, then you’ll have to cut thrust by one half.
Spacecraft are usually severely mass limited so less thrust and more energy consumption in exchange for better mass efficiency is usually the right tradeoff.
