I've upgraded over 150 computers and ran into this issue on about 9 of them. The easiest way I've found to get windows to upgrade is downloading Windows Repair Toolbox going to the 'Malware Removal' tab, Checking WinRepairAIO and Ccleaner, then running 'Unattended Fix!'. After that the installation through windows updates should work otherwise try the media creation tool provided by Microsoft. Hope this helps!
I enjoyed using D7II but wasn't quite doing enough to make the monthly fee worth it, plus CAD tanked vs USD so it was even harder to justify.
I've been using http://www.windows-repair-toolbox.com/ as a replacement and it does most of what I need.
I used Techwaru for a while too. What I didn't like about it was the lack of fine tuning the jobs that would run, d7II was much better for that. The beta just dropped for Techwaru and I plan on checking that out very soon.
If possible maybe drop wsus offline on the machines and use that with its auto restart option on. Its the closest tool I can think of that would do the job you want (windows updates) automatically.
Edit: Sounds like you should look at some RMM tools like max focus, nable or one of the many others.