Fantastic, great post. About how big is the final solution? I'm probably being optimistic but I'd like it all on a 4 GB USB stick.
The WinAIO Maker have the USB Tool for setup folders, after all, unpack the ISO and make an bootable by folder.
OK, I believe it was caused by the drive not having enough space to create the ISO. Cleared out some junk and it's worked. With HDD prices soaring a lot of people are tight for space, you should add some exception handling for insufficient disk space.