There is also play between the output shaft spline, and the aluminum servo arm. Use some blue thread locker (Loctite) between these two components. Yes, it means to get the servo arm off in the future you will need a screwdriver to pry it off (gently).

Plus the nylon gear 8411SA is a good option. Do worry about the nylon gears, the metal gears aren't indestructable either. In fact the inner pinion gears are only a press fit into the outer ring gears, and could slip under severe load.