1. Use your initial treatment plan...hopefully it has SMART goals? Stick to it. Talk to your patients about problems/lack of progress for each goal. Revise as necessary with the patient. Fire/refer patients who are not invested in treatment after reasonable attempts.
2. If this is not adequate for a typical psychologist's outpatient practice, it may be the wrong level of care for the patient? Or maybe not a time for them to be in treatment at all?
3. OQ-45 (or something with some ecological validity) seems reasonable as an adjunct to the above.
4. Measuring "progress" by using strict DSM symptom criteria (e.g., GAD-7, PHQ-9) seems fraught with clinical (and legal) problems to me?
5. The idea of conducting empirically-based treatment of psychological disorders is that your treatment is based on a scientifically accepted explanatory model of the disturbance, and that treatment is adjusted to account for progress or lack of progress at appropriate intervals. That is all. If you are conducting a research study, its different.