xTool D1 Pro Laser Focus

Use a ramp test or try focusing at various distances to confirm/optimize focus. You could attempt focus at -2, -1, 0, +1, +2 mm distances where 0 is your default focal distance. Confirm that the 0 mark is indeed the thinnest line.
Use the lowest power required to make a mark and burn to a flat uniform surface.
Before doing so, make sure your lens is actually properly clean. Use IPA to clean if required.