Ok so I MAY have found a way to do it!
So I was playing about the other day in affinity and got an Astro image that I had and copied it so there were 2 exact images then opened them both up in affinity. With the 1st one I used the selection tool to select the foreground and edit that. Then I opened the 2nd image and selected the sky/stars and edited that. Then I copied the sky from the second image and pasted it onto/over the sky of the first image and it seemed to work. I’m not sure if there’s an easier or better way but I did it. In reality there would be stacked images of the foreground, image 1. Then there would be tracked and stacked images of the stars, image 2. Then using the method above it should in theory work the same! Please let me know if it works for you. 👍