After combining the Ha, OIII, SII to an RGB file and play with levels to get somewhat desirable colors I usually apply first Filters>Noise>Dust&Scratches in the range of 2-4 pixels. Then Filters>Blur>Gaussian Blur around 2 pixels and the picture looks like this:


Then I switch to Lab Color in Image>Mode>Lab Color.
Once in Lab Color I copy my luminance image to the lightness channel. The luminance in this case is the Ha image with some Smart Sharpen or High Pass filter applied for sharpness. The Luminance looks like this:


Best,
Pablo