What is the basis of your belief that plugs should be pulled at 25 hours rather than 50? Have you observed any difference between a plug pulled at 25 versus 50? If yes, what was the difference? If they’re getting dirty or full of lead gubers in 25 hours, the engine’s being run too rich for too long.
The point of leaning an engine during ground operations is to reduce the risk of plug fouling due to an overly-rich mixture. It can be tricky getting it right on some carb’d engines because the setting is sometimes close to ICO, but with practise it’s easy. The point of “aggressively” leaning is to avoid the possibility of the engine being able to produce much more than idle / taxi RPM. Full throttle/RPM with the engine ‘slightly’ leaned can be ‘bad’.