If you want to judge more entries without increasing the work for the judges, you need to scale the number of judges with the number of entrants, i.e. use peer review. Of course if most entries are low-effort, you'll get a lot of low-effort reviews. But if you have an inner circle of trusted judges, you can have them look at the winners according to peer review and, if they don't meet the bar, throw out reviewers who rated them highly. Repeat until you've found a subset of reviewers who are taking the task seriously.