How can I collect popular questions from my students?
At one time I had a lot of success running a private wiki for my courses. Students could edit pages as is typical for wikis. I could distribute material, etc. But the advantage was that the wiki lived on from year to year, building up a resource. 

There are a lot of servers, though they require a bit of management. See https://en.wikipedia.org/wiki/List_of_wiki_software. Work with the university's IT department to deploy one of them, perhaps. There may be security concerns they can help keep track of. 

But a simple email list server that subscribes students and course staff gives a way for students to ask questions at any time. And an answer you give to one student will be seen by all. This can help you avoid repeat questions from individuals. 



There is more at this link than you ask about here, but notice at least the advice about short videos rather than full lecture videos.