First of all I can't believe that there was not even one decent junior dev among 400+. Last year I was in the same position as you but received far less applica

First of all I can't believe that there was not even one decent junior dev among 400+. Last year I was in the same position as you but received far less applications (15-20). 3 of them was decent, one of them we employed and he turned out much better than on paper. Main thing is to give them homework: not more than 2 hours project that they will solve at home and commit to private repo. Make it unique enough to avoid stackoverflow copy-pastes. Do not give them academic problems, ask them to create real world examples. For example: create CRUD that does something specific like file uploads with resize or similar; access unreliable API and do something with it; create calendar that allows multiple tasks per day (UI/UX); scrape data from the website and put it in to DB. Those are just examples, but they are easy enough to do in under 2 hours and you can test results in under 2 minutes. Out of those 400+ applications, half of them will not even bother spending 2 hours on a task, then in a day or two you will restrict that remaining ones to about 50-60 just by checking results of the task. Rest are just personal interviews. Just remember to write a feedback email even to those unsuccessful applications.