Every microwave I’ve ever looked at from a distance, or actually used, has the ability to set the power level, typically in 10% increments.

The typical workflow is: power button > number button 1-9

My daily driver is a bit more fancy, and has a single button to cycle through the available power settings.

For your milk oats I’d suggest a power setting around 30 to 40% and increasing the cooking time by a factor of 2.