Non professional or student oil color has more fillers and less pigment. Can also mean that the hues are derived by mixing pigments. This means as more tube color is added to the palette the desired color is weak, unobtainable or unpredictable. The benefit of student paints is that they are less expensive.
Insisting on holding a brush in a particular way makes no sense. A brush is a tool designed to be used in various ways to achieve a different stroke/line/pattern.
If I am standing in front of a figure model, I first need confident gestural lines using my shoulder. Applying subtle gradations of shadow to the same figure calls for repetitive wrist action. The same for fine details in focus areas.
Canvas size plays a huge role too. A broad, shoulder powered, gestural sweep would be lost on an 8 x 10. As would a 30 x 40 covered in wrist powered detail.