Advice on complex background removal?

[david]

I think that however you do it will take time. However, a possible suggestion. Use G'MIC Inpaint to remove the tower from a copy of the image and then put that copy into difference mode. From this you can either derive the picture of the mast or make a mask to use on the original.
The reason I suggest this is that masking does not need to be that precise for Inpaint to synthesize the background.
david.