>> This PDF conversion is just an Achilles heel for Moodle
I agree with that, we're actually using Google Drive and Unoconv but there are some files making all kind of trouble (CPUs at 100%, cron executions consuming GBs of RAM...) and that's the reason I'm trying with OneDrive (as those troublemaking files are often Microsoft's Office files, Microsoft may get them converted correctly).
BTW, we have the Google Drive converter working (on 3.5 and 3.7); we set it up following the docs, but if you have any specific question perhaps I can help you...