Converting uint8_t to String - unexpected result
You should convert your number uint8_t into it's ASCII representation before appending it to your string, otherwise you are appending just one character using it's byte representation.

To do so you can use a standard function itoa() which has a decent documentation available here.

So change line

 String Sync = "PaletteBrightness:" + PaletteBrightness;


into

 char buffer [4];
 String Sync = "PaletteBrightness:" + itoa(PaletteBrightness, buffer, 10);