For this specific shot, the most useful fix is careful noise reduction in post, especially chroma/color noise. Strong color-noise reduction can clean up the magenta/colored speckling without destroying as much detail as heavy overall blur. Many RAW converters and noise-reduction plugins let you target color noise separately.

Boosting contrast can make noise look much worse, so apply it gently.

For future shots, stacking multiple exposures is one of the best ways to reduce noise. Software such as DeepSkyStacker can align the stars and average the frames, which lowers random noise. Dark/flat frames can also help. With a landscape foreground, stacking gets trickier because the sky moves relative to the land, so you may need separate treatment for sky and foreground.

On exposure: at the same shutter speed and aperture, lowering ISO alone would not give a cleaner file; it would mainly make the image darker. In your case, 30s at f/4 was likely already near the practical limit before star trailing became an issue, so there may not have been much more you could do in-camera from a single frame.

Overall: reduce color noise first, ease off contrast, and for best results next time, shoot multiple frames for stacking.