[FoxForceFive]

Won't it speed up the content to 59.9 fps?

I thought it's best to play content at the native refresh rate.

I need to leave switching on because I need 1080p to display at 1080p and switch the TV to 1080p so it upscales to 4K by the TV doing the upscaling I believe then switch the TV to 4K mode when I play a 4k file.

I have the Kodi settings resolution set to 1080p 59.9hz and the firestick settings at auto 4k 10 bits RGB

It is always best to play content at the native rate, you're correct, the reason I suggested not doing is you'd said that you couldn't play 23.9 without judder, so that would be the easiest solution as you don't need audio passthrough.

Regards your firestick settings, I believe YCbCr formats are the better choice for 4K sets, but I'm not 100% sure on that, some others will know better, it may well depend on your set. Similarly with the 10 bit setting, my tv does 8bit +2 instead of real 10 bit, so I have it set to 8 bit, using the logic that 8 to 8+2 makes more sense as there's less processing involved. Again I may not be correct on that though, someone else more knowledgable may correct me on these matters.