I should probably preface that I'm no expert in astrophotography. I kind of brute force things, so my recommendations may not have any basis in actual sound advice. 😉
I mostly use 400 speed film, so I'm afraid I wouldn't be much help for specific film recommendations. I do think there's probably a trade-off between finer grained slow films versus exposure times (accounting for reciprocity). As for f/8, if I can't set up a composition before night sets, the viewfinder is pretty much black. My thought is that stopping down to f/8 gives ample depth of field while keeping any foreground elements reasonably sharp and in focus. Mostly, that combination keeps me from overthinking things and just being free to see whatever comes out.
