"Create Diagnostics Report" not generating (or not generating in right location)?
I just ran this on my older iMac running High Sierra. It ran to completion in about 10 minutes. It wrote the output to the tmp directory but probably not the tmp directory you checked. On my machine, it wrote the output to /private/var/tmp. The folder 'private' won't show up using Finder unless you 'show hidden items'. This requires hitting the command-shift-dot keys at the same time. Then the Finder will display the 'private' folder. So from / you can see the private folder. Other folders on that level are Applications, Library, and Users among others.