You need to know the intention of the basic strokes for when you begin to write the letters of the alphabet. You must be able to deconstruct the letter in order to understand how to build it. The good news—now that you are able to make basic strokes—you can build most of the lowercase alphabet.
As you peak just above the median line, begin to turn your fingers inward and apply pressure. This pressure should be the most you apply out of any downstroke you've previously practiced because the broad edge is going to be less generous as you move in back towards the left.
For thin upstrokes, use the tip of the marker with little finger pressure. This might sound crazy, but grip the barrel as if you are picking up a baby bird with your thumb, middle and index finger. I know you're an experienced hatchling caregiver, so imagining this shouldn't be too difficult.