I never thought to do this before on my comet situations; I always just composited and cranked exposure down. This might be image-changing for some of my old exposures, so thanks for responding.
You can also import the starless image in Photoshop and play with Dust & Scratches filter that can remove all the trails.
It will also remove the comet details, but you can always mask and retrieve it back with History brush.
Eventually, you’ll be left with Comet image that needs to be stacked on top of the star image (assuming you have already made star image without a comet).
I’d up those faint stars and sky to not be that pitch black, but that’s personal.
Great, you managed to catch a tail at least. I haven’t got a tail on my images. I used 135mm after all.
But I’m super happy because I literally had like 20-25 minutes to work out of 2 hours I spent outdoors, as the clouds were just coming. I had a few of those cracks of clear sky.
Even if my result is not very different from the previous ones the fusion of levels with Gimp in this way is more easier (anyway remains a bit of a sort of shadow of the trails of stars after the stack of files"starlessed") .
Instead of Gimp you can also use Siril directly with Recompose stars and Gimp only after.