I don't understand why your wheels are having so many problems so quickly? My Santiagos are far from mint with a few areas of kerbing but no signs of whiteworm after 4 years. Is it a local conditions issue? Perhaps getting them refurbed by a quality company (Lepsons are regularly recommended) they will last longer? If you are determined to go down a painted finish, how about keeping them as is ie with the diamond cut parts painted silver?
I think it’s likely to be quite expensive to have them refurbished in a two colour paint / powder coat finish to replicate the original diamond cut finish. To achieve a diamond cut look with two different colours of paint / powder coat it’d be necessary paint / powder coat the entire wheel in one colour first, and I suspect that accurate masking each wheel to cover those parts that are to remain in the first colour before applying the second colour would be quite time consuming (and time costs money). If short cuts were taken on the masking up, the end result could be paint bleed under the edge of the masking tape, and crooked lines between black and silver sections on the wheel.
That’s why these two tone wheels are almost always diamond cut - it’s much quicker / more time efficient for the wheel refurbishing company to paint / powder coat an entire wheel and then remove paint from the face by re-cutting the face back to bare metal on a lathe than it would be to paint, mask up, and then repaint.