In general, if you're using reactionless drives, you're throwing realism out the window, and they also don't actually do a great job of emulating most fictional genres (there are edge cases like The Forever War), so I find it simplest to just remove reactionless drives, and replace with techs that do a better job. The usual candidates are:
- Ether Drive: the drive works by pushing on the ether, or something similar. This means it has airplane-like performance -- deceleration and turning are relatively easy, but acceleration drops sharply as velocity increases (power requirements are equal to the dot product of force and velocity). Obviously this is throwing relativity out the window.
- Planetary Drive: similar to an ether drive, but instead of pushing on the ether, it pushes on nearby large masses. This adds an additional limit on acceleration, as it is also reduced if there are no nearby large masses (a reasonable cap is some multiple of local gravity).
- Sublight Warp Drive: it's just the setting's FTL drive, tuned down to be slower than light. There are reasonable energy arguments for FTL drives having a max speed that is inversely proportional to local gravity. This has the useful side benefit that speed is much lower in the areas that are most interesting to be in -- a drive that can get anywhere on Earth in an hour can get just about anywhere in the solar system in eight hours (oddly, Mercury is the hardest to get to).