I’m not very experienced with capturing star trails. But I can try to offer a few tips:
Experiment with your camera a bit to find a combination of exposure time and ISO that works well.
At least with the cameras I own that’s not ISO 100. Shorter exposures + stacking work better. ~ISO 400 + ~1 Minute exposures would be a starting point but that of course also depends on the aperture of your lens. Stop down a bit to reduce optical issues.
If you are using a DSLR don’t forget to close or cover the viewfinder.
Capture darkframes both before & after.
Err on the side of under exposing rather than overexposing. With 14 bit raw files you can afford to lose some bit depth. Clipping on the other side is mostly irreversible and will make your image look flat (all the stars end up at the same brightness)
For stacking there are multiple approaches, ‘lightest’ seems to be very popular for star trails. For the foreground try a mean or median.
I hope at least some of these are helpful.