Photoshop Soft Brush banding problem

I am having a problem with my soft brushes in Photoshop CC'19. I am trying to clean up the background of a portrait photo, but the soft brush is creating some obvious banding. If I colour over a simple white background, the brush paints on with a smooth(ish) gradient. As soon as I put it in my dark grey background, the banding is obvious. Here is one example below. The same black layer over the top of a white background, and then removed. You can see that the one on the pixels has unattractive banding compared to the white.

Here is the photograph which I am trying to edit (lightened to see the banding easier).

Above you can see that I have added some colour to the edges to create a landscape photograph. I am trying to blend it in to the photo so there is not a harsh edge.

Below, you can see that the soft brush is creating so weird jaggard lines/banding..

Here is another screenshot of another attempt.

This problem is really frustrating me. Any help would be appreciated!