Good work! I'm surprised you were able to get that much out of the kit lens on an APS-C camera, although the Nikon 5600 has a really good sensor.
Few tips (I'm a semi-pro landscape photographer):
1. Instead of using levels or contrast or curves to adjust the "pop" of the galactic core of the Milky Way, try using a unsharp mask function. I don't know what Gimp has to offer, but if there is an un-sharp mask tool, then use it at a super high radius setting on the sky, then adjust the amount of intensity to your liking. This is the easy-cheat way of getting a nice pop out of the galactic core without completely nuking the rest of the image into clipping to black.
2. For an APS-C sized sensor camera, you ideally need a lens as fast or faster than f2.8 to be able to do much editing on the RAW files. The stacking software helps to mitigate this, but it has limitations.
3. Try 30 second exposures at ISO 800, shooting in RAW. The sensor has plenty of ISO invariance that by ISO 800 you can push the file brightness in post to ISO 1600+ without any additional noise compared to shooting the RAW files at ISO 1600+. What you get back by shooting at ISO 800 is added dynamic range which helps retain the colors in the brighter stars and areas of the galactic core.
4. The major drawback of using the stacking software is that the edges and corners of the final image once stacked together will be very soft and blurry. This is due to many factors, but mainly because since the lens is stationary, it's combining areas of the sky from different parts of the image frame. With wide angle lenses, this blends the distortion and lens artifacts such as coma. That explanation probably doesn't make much sense if you are new to photography, but keep in mind that too many exposures is detrimental when using this technique.
5. 65 shots is way too many for your setup when stacking from a stationary camera. Unless your have the camera tracking the sky movement, try keeping your stack to less than 10. If you use longer exposures like 30 seconds you will get a bit more streaking in the stars, but that's the trade-off when shooting a slow lens on a crop sensor. If I was in your shoes, I would shoot maybe 4 frames at ISO 800 for 20-30 seconds (depends on level of light pollution).
6. If you really get into doing this as a hobby or semi-profession, then it's worth learning how to use Photoshop, which is $10 a month. Despite what many like to shout about on the internet, nothing comes remotely close to Photoshop when doing more advanced photo editing. The stacking software is still nice in addition to Photoshop, but software like Gimp and Affinity are mere clunky skeletons of what Photoshop can do.
7. One major drawback of using the stacking method with multiple exposures is that the ground will be blurry in the final output because the software is aligning the sky, which moves. You will need to learn how to do a separate exposure for the ground and then blend it manually to the final sky output image. You can then use any shutter speed and aperture you need to get the foreground properly exposed and in focus.
8. Also look into doing panorama stitching to widen the angle of view. 18mm is not very wide on crop cameras and shooting 3 or more images in a panorama will not only increase your view angle, but it will also reduce the noise of the final image, because you are in effect gathering more photons and using a larger sensor for the capture. It only takes about 3 shots in a panorama to move an APS-C sensor up to the realm of full frame sensor in terms of noise control, given the exposure and aperture are the same.
Also, try joining a photo forum like Digital Photography Review or Fred Miranda, where you will find countless people willing to help, and especially those who are using the same gear.