There's no rush to do everything at once. You can edit what you've done to your liking, and then continue adding more strips. Strips can be joined at their ends using Step Build or Bridge, or Target Weld. You can also use the Drag brush
to slide the points of the strip on the high-resolution model surface.
I like to use the Strips method to establish the symmetry line as much as possible all the way round the model. You may want to skip over areas of tight detail like the mouth, saving them until later. Sometimes the strip's edge distribution will be a bit fine, as in the example shown below. Consider using Dot Ring to grab every second edge and choose Ctrl + Remove to lighten the mesh.
You will notice that since we're adding the strip along the symmetry line, there isn't a good fit to the actual middle as the polygons don't have a center edge. We can add a center line by making a Ring, selection around the strip, then ConnectEdges to make a new loop. To force this loop to be centered...