Disgusting outline on mask when attempting to change background elements

'Using a brush to manually alter the mask is too painstaking...'

If you want a professional result, then it is very rare that a mask is a one click process. Whether it starts with a channel mask, select subject or any other technique, a good mask requires work. That can be manual brushing or combining the output from various 'one click' masks. For example brushing with black/white and the brush blending mode set to overlay is a good way to sharpen up selected areas, conversely, the blur tool can be used to soften the mask in specific areas.

When working on the mask, continuously switch the view so you can see the impact against the new background (or in your case adjustment layers). That way you can focus on the areas where the mask is unacceptable without spending hours on areas which are not really visible anywhere else but the mask itself.

Dave

@レオンマ 

You may have an compounding effect that is making things worse... Try creating a group with a single layer mask for the subject and remove the multiple layer masks for the various adjustments and move them into the masked group.

Not sure what you want to ask, but remember that you can use various options from the menu to modify the mask, such as filters and destructive adjustments like Levels and Curves.

This is why an automatic algorithm can never replace a real human.

This isn't difficult, Photoshop has all the tools you need to do it. But you have to put in the work and use your eyes. Making appropriately blurred areas in a mask is straightforward with soft brushes.