i noticed that my gigabyte aorus rx580's bios seems to be limited to 1500 mhz.
What kind of software is that on the screenshot? And btw any Windows OS has a built-in program called "Snipping Tool" which makes proper screenshots instead of that mess of a shot you attached.
You might want to use that in the future. For flashing Polaris cards all you need is atiflash to save the bios and flash it back, and PolarisBiosEditor to edit the timings, clocks and voltages. Can you take a screenshot of your bios opened with it?