Burning at a slow speed doesn't necessarily result in a better burn quality, which is what the reader devices care about. As you've said yourself, that's 8x rated media, so you should at least try that and 6x.
By the way, it's the burner who determines the available burn speeds, and while ImgBurn sends the command for the 2.4x write speed you chose, the burner simply ignores it and uses the closest available write speed, which is 4x. If you want to burn slower than that then you'll need to find media that your burner supports 2.4x with.