“Given that, wouldn’t the top speed of any interstellar craft be limited to, or less than, the speed of whatever is coming out of the tail end of the craft?”
Well, what needs to be equal is not the velocities but the forces, and the force equals the mass times the acceleration.
So, a spacecraft generally shoots particles of very small mass at high acceleration out the back of the ship, which generates an equal force towards the front of the ship. However, because the ship is much more massive than the particles of propellant, the acceleration of the ship is much less than the acceleration of the particles.
Now, since spacecraft operate in a vacuum, the velocity will increase as long as they maintain that acceleration. Really, the velocity is limited by practical limits, like how much fuel the craft can carry, or whether the craft gets assistance from gravity using maneuvers to accelerate. As a certain point, there would be a hard limit to acceleration, as you approach the speed of light, but we can’t get anywhere near that, so it’s pointless to worry about.
Cool, thanks for the explanation.