In flight, at 65% power or below, you cannot harm the engine with the mixture control. If you have an engine monitor, learn to use it.
If you have a carburetted engine, use of a bit of carb heat may even out the mixture enough to make it run lean of peak smoothly. If you have a fuel-injected engine, it may take some injector swapping (with the help of competent people) after doing Gami lean tests. My engine now runs smoothly LOP, the last straw was the change of spark plugs from Champion fine wire to Tempest.
The engine management and in particular the use of the mixture knob at PPL level is vastly underteached.