Surely the solution is that a ship can only fire on an enemy ship it can "see", either visually or by radar. Otherwise we are pretending WW1-2 era ships have modern datalink systems, and that flies in the face of any pretence of realism.
21 knots for the QEs may have been because the rest of the battle fleet could only do 21-22kts? Hood's gunnery officer reported that above a certain speed (mid 20s I think) vibration in the spotting positions/DCT became "excessive", presumably making accurate shooting more difficult. I think I remember reading something similar regarding vibration on Invincible and Inflexible at the Falklands too.