darken the sky

[ApvdG 10]

I want to darken the sky compared to the lower part of the picture, and not having a sharp boundary. I was trying to do this by using a gradient mask. Starting from the original picture I added one adjustment (levels), then press Alt and click on the adjustment layer (= white), and then select the gradient tool and create the gradient. Works very nice, only affecting the sky, and no sharp transitions.

However, if I add a second adjustment (e.g. exposure), that adjustment is (obviously) affecting the whole picture again.

I have two questions. How can I create a gradient mask that works on several adjustments, and how can I do that in such a way that I am able to modify the initial gradient at a later stage, using the same interface as when I create the gradient in the first place (so retrieving the line with the end-dots, to then just moving them slightly)?

[HuniSenpai 16]

Ah, a good ol' graduated ND filter  

You pose a really interesting question here. From my understanding, in order to make the gradient modifiable in the future, you must put that gradient on a fill layer. This is difficult of course, because Affinity does not seem to let fill layers function as masks. Even if there was a method to do this by using fill layers, it would be far too over-complicated to make it convienent. I'd be curious if someone else has a suggestion on how to do this. 

Anyways, here's everything else you asked for (I think )