As someone who went through the application system to join my instance, it is really not that bad. Just have your potential users write a few sentences on their lemmy history, why they want to join your instance and what content they are looking for.

This can be answered in a few minutes, but it is already enough to discourage almost all bad faith applicants. The few hours of wait period while you check their application for copy pasting are also contributing to deter them, simply because spam users often want to do some mischief right now, waiting until tomorrow doesn’t work well for those impulses.