Hi, this is one of the first things I noticed when taking the raster brushes for a spin. It is impossible to work without getting rid of the brush preview. A single pixel pixel or a cross hair is what's needed. I am used to concepting in corel painter or Photoshop - just a cross hair or a pixel - the less obtrusive the better.
As a long-time user of Photoshop (boo down with the cloud looking for an alternative) and Corel painter, one feature that Affinity designer would benefit from is the “lock layer transparency” tick box option. I have only played around with Affinity Designer for a day – there is probably a way of doing this with masks, however, I use lock transparency layer alll the time for blocking out silhouettes then paint within the layer. Also there is no support for TGA’s format in designer or photo – still widely used in the games Industry for texture creation.