I'm an STS student, so bear with me; I'm happy to provide clarification but I want to leave the specific application unspecified:
What sources would you consult if you were trying to generate a huge "list of all academic topics"?
I'm thinking it would include all the 'ologies', the 'isms', the 'X studies', all the historical "movements", all the 'subjects' of journals and papers, all the 'big questions' of any given "field".
What other sources, whether vague (like above) or specific (like a URL with some good list items), would you suggest in compiling this list?
You need to define topic first. At the macro level, all topics fall under "knowledge", or some such nonsense, and at the micro level, every published paper and book is on a different topic.
You could use the different doctoral degrees as pretty broad topics. This would give you a list like philosophy (PhD), medicine (MD), science (ScD), etc. A less coarse division would be "schools/College". This would give you a list like School/College of Arts, School/College of Humanities, School/College of Medicine, School/College of Science, etc. A finer division would be departments and institutes within a School/College. A still finer would be research groups within departments. This of course leads to individuals within groups and finally publications by individuals.
University websites would provide all the information you need, although not in a format that is easy to search and retrieve.