I also have a set of "Pre Takeoff Vital Actions" that I perform, although normally just before crossing the stop bars, as after that point I want to have my eyes outside the cockpit as much as possible!
We lean for for taxi and lean for cruise but don't touch the red knob between "Power checks" / run up and takeoff.
Would you not set the mixture to the correct level during the runup then not touch it?
It would have to be a very long weight for plug fouling to occur I'd have thought?!