Brush painting is about 60% learned skill, 30% tool and paint quality, and 20% art. Just do the math………
First of all, use good quality brushes and maintain them properly.
Second, use good quality paint. On this, I will not comment in your case—you have to use what's available. Haven't heard any serious complaints about MM enamels, anyway.
Third, paint on a reasonably clean surface. While enamels do a better job of dissolving skin oils and form release agents on the plastic surface, getting rid of these will make painting much less frustrating—you're removing variables, and that's always a good thing.
Paint in an environment where the temperature is cool, or raise the relative humidity, or both. Raising humidity also helps eliminate dust problems.
Decant a small amount of paint and add a few drops of a slow evaporating solvent (mineral spirits is a good choice). On a piece of clean scrap, try painting a few strokes. If the paint levels, it is the proper viscosity, if not, add a little more thinner. Repeat until the paint levels. At that point, you will know what the right viscosity looks like.
Don't try to get full coverage with one coat. Count on at least two, and follow recoat times recommended by manufacturer. If none are given, allow paint to cure fully before recoating (overnight to 24 hours to be safe.)
PRACTICE, PRACTICE, PRACTICE!!!