I do that for normal nuts n bolt plus an identical range in St/steel….from 3mm-20mm
bought a few 3,4,5mm screws of diff lengths with nut's and nylocks also the same and in st/st…plus self tappers..
along with a big range of Allen bolts……
but this stuff gets used and replaced reg …dread to think of how much money is invested there….
as for ur rather special wants do as above, luckily they dont cost much in those sizes and after a while u'll get a feel for what you need….
then order enough for the job plus 50% that way ur always ahead…
as for the end finish u can always machine a flat from a pionty end….
Find a good local supplier if u can but buying on line is so easy now….
I reg used a fastner supplier in Colchester for specials, who would post to France no probs….
wether I wanted 5 or 50…