You've been advised correctly - thinking a bit harder the technique I've suggested is trying to cope with too much light for a long exposure, not too little.
It might be possible to stack images and use an additive mode for each layer. I've read that lots of asto photographs are stacked form multiple images but that's the sum total of what I know
It would be possible to test - set up a night sky/star shot on a tripod and find a sensible length of exposure to get a bright image.
Then take a series of shorter exposures that add up to the same exposure - for illustration One 1 second exposure = Ten 1/10th exposure frames.
Import them into GIMP as layers and change the mode - I'm guessing "Lighten only".
My knowledge of astrophotography is next to nothing - if you need a 3.5 second ISO 6400 exposure (as the image above) then you're going to need a lot of images to freeze any ship motion induced blur in the stars and still get a cumulative 3.5 seconds
If 1/10th was fast enough you'd need 35 images.
I know from my limited stacking experience in GIMP manually aligning 35 layers for rotation and translation is going to take ages and drive you nuts if the layers themselves are heavily under exposed so there's not much to use as a marker (e.g. a bright star or lit object on land).
I'm way out of my own experience here so probably just making things worse, sorry.
You probably need some very effective stabilisation rather than attempting this.