masks – How to masking a picture in Affinity Photo

I have the following picture. Using Affinity Photo (version 1.8.5.703 on Windows 10), I want to mask everything except the child and the green rope she stands on/is holding in her hands. Everything else (playing ground, houses, trees, sky etc.) should be masked.

What I tried: i made a selection, using Selection Brush Tool as well as Flood Select Tool, than removed (cutout) the selection. You see the partial result on the second screenshot.

This alredy took almost an hour since I had to set a quite low Tolerance setting for the Flood Select Tool because otherwise, it would select too much, thus also parts that I don’t want to delete. When I come closer to the rope and the child, I’m stuck as I almost have to make a pixel by pixel selection – this would take hours if not days. You can see that near the rope (red arrow) there are already pixels deleted that should be kept. The leg (other red arrow) is another difficult part.

How can such a picture be masked the way I want it?

Original picture

partially masked picture