Vapour blasting with plastic beads gives by far the best finish.
And the beads don't embed themselves in the alloy, to come back and bite you later, unlike most other forms of blasting.
Can't help with any practitioners for you, but many gearbox shops use these, ask about locally. ?
MCNews will have adverts for bigger operators.
I've used sawdust to lightly clean up alloy on whole assembled bikes.
Many cabinet makers are pleased to give it away.
(May need sieving to take out the larger bits)( plastic bags around carbs/aircleaners etc)
hth
