Welcome here.
BMW have been using stainless spokes for many a decade, and haven't detected any problems yet !
They do use them in straight pull configuration though, which is probably the secret.
Now grades of stainless can vary, considerably, so it may depend on who you trust to supply them.
They are very widely used these days - and few classics would meet really rough roads to test them out ...
Brake linings these days can come in "high-friction" varieties for some makes, so maybe these are available ?
Making sure the linings are thick enough so they can be arced to fit the drum is a good first step to getting good brakes.