If you are a collector, I think you have to assume that you have flat mites on at least some of your plants. You also have to assume that any new plant or cutting that you take in will have them. If you grow outside almost exclusively, I believe that frequent sprayings with the hose and natural predators of these mites can keep them at bay to a degree, but if you exclusively grow indoors, you will have a problem at some point.

I cannot tell anyone what they should do, but after only 3 weeks spraying everything with sulfur has made a huge difference for me. The number of Hoyas putting out new growth that have not grown for a very long time is remarkable. A couple of examples: H. serpens is throwing out crazy growth from many points when it just sat there for months. H. sp. Timor Leste had not done anything for months and had some yellow spotting on the leaves, sending up new leaves and vines. The same thing can be said for H. sp. Gunung Gading, H. undulata, and H. ruthiae. I would say that 90% of everything that I had that was either not growing or growing almost not at all is throwing out new growth. Below a photo of H. serpens that had not grown in months, and one that I have greatly struggled with over the past few years is busting out with new growth.