Sorry I don't have an answer, only more questions on this. I also read about this limitation. Perhaps it's more of a warranty issue and direct sunlight exposure merely reduces the lifespan by a few percent. Given that you have been using Waveshare displays a lot already, what are your observations on this? Is it really problematic?

I've been using an ancient ebook reader for reading recently, and contrast will be washed out to about 50% when exposed to sun while page flipping (until next refresh). However, I've been thinking this is due to the old age of the device.