When running Disk Clean-up, did you run it as an administrator? When you run it normally, there will be a Clean up system files button on the bottom left - this is usually an indicator that you didn't run it as an administrator, but clicking the button has the same effect.
Have you upgraded Windows 10 in the past? If so, there might be upgrade files remaining that you could probably safely delete.
If possible, post screnshots from Treesize free. I'm gambling a large WinSxS folder in the Windows folder or forgotten about upgrade files, like Windows.old.
You might be able to get the size down further using some DISM commands.