I am now confused, in that you do not download the Windows Update Assistant (not in the conventional sense, anyway) you just kick the process off using a button on the Windows 10 Download Page.
If that's where the issue is arising, then I would use the ISO method, and I suggest being disconnected from the internet during the update and, when asked if you want to download the latest updates, answering, "No."
Windows 10 will take care of all of those updates via the usual Windows Update process after internet connectivity is reestablished after the version update is complete.