Hi, no help on the powder coating I'm afraid, but in the same boat as you albeit just the five Cosmics to refinish. Went through the same cost/benefit analysis as you did with the idea of getting them diamond cut, only to find I could have bought a new set from Cosmic (or whoever holds the IP for them now), plus I'm trying to do as much as I can, and this will be the 9th or 10th set of alloys I've refurbed, including a set of Cosmics for my Mini half a lifetime ago, so hopeful of an acceptable outcome.
Only thing I can offer which you may already know about is that you can get scotch pads for rotary sander/polishers, which I didn't know about until I watched a few YouTube wheel refurb videos for tips. Ordered a set which arrived this week so not had chance to test them yet on the spare, they are marked 3M (but came from China so who knows) and might reduce some of the sheer drudgery and fingerprint recognition fails that follow doing it manually. Will post again when I have tried them out.
https://www.ebay.co.uk/itm/224123446309?var=522912300925