But people could objectively measure clients' subjective experiences captured on tested, reliable standardized forms

Then the aggregate could be looked at and compared with other therapists, say-in the same clinic/organization. One who is a consistent outlier could be a crappy therapist.

Questions-
Do you feel respected by your therapist?
Do you feel you've made positive changes to your life as a direct result from this therapy?
Do you feel supportive?
Do you feel safe with the therapist?
ect

and
Do you feel anxious
Do you feel hopeless?
Do you sleep well?
Do you eat well?
Are you able to perform your daily taks
Are you isolating?
etc
Every day, most days, somedays, hardly at all, not at all

Unlike research, you can just use basic statistics. Look at differences among length of treatment, diagnosis, treatment modality, etc. But have an overall score for each.

If someone's patients answer they don't fee respected, while other therapist's have average answers or above, that could at least identify the therapist might be a jerk.

A client can take a short 60 second questionaire after every visit too about how they feel about the overall visit.

Since depression and similar subjective expereiences, that doesn't meant they can't be assessed objectively