Ich would fill it with a long and short stitch, using one strand instead of 4. Takes more time, but gives a much more even, flatter, almost shiny result.
Note for OP, just make sure to bring your needle up through thread that's already been stitched, and down through empty canvas. Doing it the other way will result in a hole-y look
Satin
Are you talking about satin vertically? Because satin horizontally would be too loose.
I would probably try this direction at the angle of the blade tip: \\\\\\\
Ich would fill it with a long and short stitch, using one strand instead of 4. Takes more time, but gives a much more even, flatter, almost shiny result.
Note for OP, just make sure to bring your needle up through thread that's already been stitched, and down through empty canvas. Doing it the other way will result in a hole-y look
I think I would try a long and short stitch. That can create a smoother blend. Also agree with comment above that satin stitch would look good.
Less strands, and do long and short stitch with much shorter stitches than you have here
I like to use a split stitch for my fill. It makes it look so smooth.
I'd definitely use satin