Simple solution to the “no engine” tankers, just merge Engines and Powerplants so that Engines produce the power for the ship. This way to get the power you need to run the rest of the ship, you have to put in engines.
After all, Kirk always called the Engine room for more power, not some separate powerplant room.
IMHO, have each engine produce about the same amount of excess power for the rest of the ship, regardless of how much thrust it provides. So a low-thrust, light-weight engine produces nearly as much as much excess power as a high-thrust, heavy engine. This will further encourage quicker ships since you’ll have to pack in engines to get all the power you need on a big cruiser.
Perhaps “left over” power is used to improve thrust (“More power to the engines!”). So you can make fast ships by putting lots of big engines but not using all the power.
It would also simplify ship design since you don’t have separate power plants to worry about.