I would think taking a ton of in depth quality workshops/continuing ed and paying out of pocket for ongoing supervision from someone good would be best bang for the buck.
A doctorate program would make you stop practicing under your current license while in training. Since you are in private practice, goodbye income. You will also spend years learning about things you apparently won't get much of a direct ROI on. PhD/PsyD programs will also require several moves, often across country, if not for the program itself, for doctoral internship and postdoc. It is a 5-8 year full time committment and reduction in lifestyle.
Edit: What about taking or auditing select course at places near you? You'd probably need some legit connections to make this happen at a non diploma mill place.