I do 1/1000 if I can or 1/500 but that's at f/1.7
that's when I started getting sharper pics. They would appear properly exposed but the speed helps the blur (this all non tracking mount)
Basically I've found ,do it as fast as you can cause its /we are moving faster than you think . Def timed or remote shutter release!
I haven't had a good star trails shot yet but was trying the smallest aperture I could get and as long as I could expose before overexposure.. Was doing 2-3 minutes... But I'm still learning so this is not advise
I recently saw a great pro one and it was a composite of multiple short(ish) which seems a good idea to try.. Perhaps multiple exposure of the same frame! (?)