Milk powder doesn't dissolve
Milk powder is not supposed to dissolve in fat, so you will never get a perfectly smooth product here.
The closest you can do is to disperse the milk powder well. This quite finicky. I prefer the technique where you carefully blow small amounts of the powder into a mixer running at high speed (this won't work if you have a very stiff mass). It will get rid of the lumps you describe, but there will be a noticeable graininess left. Trying different brands is also helpful, as some are milled finer than others - just feel with your fingers which is the softest.