"IE: Instead of heating something for the typical one
minute at full power, heating it for two minutes at
half power(or even medium high or 60% if such
setting available)? That way the magnetron cycles
on and off instead of running constantly, and also
the food is more evenly heated. "

No. It is doing the same work AND being thermally cycled. Now had you an older one where they put more than six bucks into it, they had a lower tap on the transformer for the lower power setting. One of those maybe. With that, lower power might just make the tube last longer. Not by cycling on and off for a longer time.