Q.Cut it down?
Downtoearthdigs, seeing it is very extensive, would you cut the bush down? After reading up on camellia scale, to treat it seems like it wouldn’t work; the bush is literally covered with it and much of the underside is already black/brown.
After seeing this severe image. I agree completely with you. I would start over.
I always say a loss in the garden makes for new opportunities. This is an excellent time to shop at local garden centers for new plantings.
Best of luck!