Just adding that this is happening all day with me. Trimmed my world to 70MB. As soon as I add some datapacks it will not allow me to upload without the 504 error. It seems to be a datapack issue from my end. All are common datapacks.
I updated my graphics using AMD Adrenalin and worked for me. Check what version you have for your pc and try to updated them.
I've updated my driver's and updated the graphics and restarted my pc, and it seemed to work! I've also gave the game some extra memory usage.
I'm having the same issue
I have the same problem but my iso is 15.8.8