Thanks for the hint. I have seen the article and also tried to do something similar before but could not figure out how to use Siril at that time.
I followed a different approach:
- I took more photos at the place with the stacking idea in mind
- I used Sequator (free but not open source) to stack 5 images to get more details and get rid of some of the noise. Also, it is able to process the sky and background separately so you can skip the blending part of the workflow using Siril.
If anyone is interested and wants to start from the resulting tif file I can upload it, but it has 140Mb, I do not know if I can attach it directly to the thread.