Reddit mentions of Diagnosis and Treatment of Mental Disorders Across the Lifespan

A previous version of this is what I used alongside the DSM to learn psychopathology in graduate school. It is informative and thorough, and covers in much more depth the descriptions of mental disorders and differentiating them, rather than just listing criteria.

Not sure if you meant this, but psychometrics don't have anything to do with diagnostics. Psychometrics is a subfield in statistics and tests/measurement that deals chiefly with the statistical concepts of reliability, validity, measurement bias, and other topics relevant to accurately measuring what we think we're measuring. If that's what you're after, this is a good resource.