My only suggestion would be to deal with approaches to problem solving, let the candidate speak, and try to gauge their knowledge with a few well chosen probing questions.

A few times when I’ve been involved in the hiring process I’ve tried to deliberately lead them a certain way and see if there’s any depth to their knowledge so they either call me out or agree and add more detail.

You’re absolutely right in saying if you have many candidates then you need some way to whittle them down for sure. A good agency is always useful if they’re on point, but often that’s not the case. Having so many candidates is a headache, is there no way to filter on minimum (verified) qualifications and experience?

Maybe you could write up your experiences, see if we can find some common ground to help people out.