I just learned how to actually apply stretching to the image in Siril, instead of only just previewing it in the preview windows. The Siril autostretch and asinh profiles are really nice, and tend to look better than what I can manually do in Gimp.
First, have your stacked and finished image open. Then, go to Image Processing → Histogram Transformation. This brings up a window that will actually stretch the currently loaded image.
After this is applied, the File → Save As option will save the image with the stretching applied.