The first link from Reddit the User is using a RX570 for his game settings.
The bottom article with the video should be good for your RX580. But in AMD WATTMAN put the power setting to 50% which helps in that GPU card stability.
Just google "Oldrim Optimized settings" and you will see other websites with more information and ways to configure the game's INI file. I didn't post it because it seems kinda technical in modifying the game's INI file and using MODs. But you were asking about Radeon settings which is why I posted what I did.
Anyway, that is all I know. Maybe someone else has more info to help you configure Radeon Settings for OldRim.
By the way, If you set the Radeon Settings according to the second article, and you have problems, you can start either enabling or disabling settings until you have a stable game. But at least you have something to start with.
Take care and Good Luck!
EDIT: about the missing settings, seems like you need to add special command lines settings to the game's INI file. You can see what needs to be added by googling "Oldrim Optimized Settings" Those missing settings you can configure in the game itself Settings.ini file. Plus those settings is for SkyRim itself which might explain why you don't have it in yours. But the articles says it should be valid for OldRim version.
