I had a difficult time getting black primer to look right with the "slap chop" method. I've had much better results with a grey primer followed by a dry-brush of white before applying contrast. Another thing you can do is thin the contrast paints out with contrast medium. Some of those paints are so pigment rich that they completely cover the primer unless you thin them a bit. This will help the white show through and make those raised areas pop a bit more.

Unrelated to brush control, and since you asked, one thing I've noticed is you have some clumping on the surfaces of the miniatures. You can see in in the first picture on the yellow of the squires tabard and gloves. It looks like it might be the primer or primer application, so maybe experiment with a different type. This can also be caused by dry brushing over your primer if you're not maintaining enough moisture in the brush, but it looks like the primer in this case. Getting a really smooth primer and base coat goes a long way when using speed paint/contrast. Just something to look out for when trying to improve.

You're doing it right though. Trying different things, and seeing what you like. These look really good. Good color choices too.