Here are a few thoughts on the role of medium and low resistance brushes (especially in regards to the task of reducing the appearance of brushstrokes.)
The Brush Wheel:
One of the early ideas/heuristics that we present to painting newcomers is the idea of a “brush wheel.” We organize this wheel, not in terms of shape or necessarily size, but in terms of “resistance” (what some people may refer to as brush stiffness.) Most approaches to painting loosely follow the 123 scenario with higher resistance brushes often capable of applying far more (or thicker) paint (often conducive to a large-to-small process.) As such, we have our students begin with the 123 approach. Over the years though, my own process has evolved into a 321 approach as my approach shifted to small-to-large.
You can actually learn quite a bit about a person’s process by how they might navigate through this wheel. Some might not even stay in one direction (shifting at different stages of a painting.)
It is important to acknowledge the difference between blending and rendering (removal of brushstrokes.) Blending involves an intentional interspersion of paints (colors) whereas rendering is simply attenuation or diminishment of surface texture (i.e., paint film topography.) This example is of a simulated Sargent painting that was “over-blended” in that much visual information has been lost, likely in the pursuit of a smoother painting surface. Here the information could have been preserved if a lower resistance brush had been employed.
Hope this helps!