About the only flying modelers who expect less than long life out of glow plugs are racers. They know that their practice of setting for max rpm flight after flight is going to result in at best a slightly too lean run every so often.

Trying for high rpm and max performance usually means that you're needling the engine as close to flat out as you can, and if you're also using props with shorter diameters, you got two strikes before takeoff.

Max plugs seem to be as good as plugs get. There are others as good, but the point is, you got nothing wrong with your choice of plugs. Next is your 10% nitro fuel. Also nothing about 10% that shortens plug life. What's left? RPMs and lean runs.

Glow fuel two strokes depend on some amount of unburned fuel taking heat out as it goes. If your exhaust isn't showing fuel in it, the engine is probably seeing temps over what the mfg planned.

The beauty of our flying is that we can land right away and give the needle a click or two richer, takeoff after takeoff. Try an experiment. Set the needle as you normally do. Next plug that goes, put in a new one of the same type. Now set the needle as you normally do. Then give it two clicks richer and fly. Every time after that, when you reset the needle, set normally for you and then add two clicks richer. Chances are good that the airplane will actually fly with a bit more power AND that the plug will last longer than the airplane.