Hey thanks!
So... funny story... I tried the sketch you linked and it worked fine. Then I tried calling the M5Dial.Display.setBrightness function in my sketch, and it did nothing regardless of what number I passed to it.
Long story short, I'm using Bodmer's TFT_eSPI library in my sketch and I had TFT_BL defined in the user config file. It seems that defining that makes the TFT_eSPI library take over the backlight control pin and stops the setBrightness function from working. Commenting out the setting of TFT_BL in that config file made the function work.