It's a great idea! I applaud your effort. 👍
The sooner you begin, the better (and the quicker you'll pay off that small loan). I recall reading somewhere that psychology interns in NM were starting their psychopharmacology post-docs concurrent with their internship. This would shave at least a year off the didactic portion of the training. If you decide to do another post-doc (non-psychopharm, i.e., health/neuro), you could complete your MS in psychopharmacology at the same time and begin your practicum and conditional prescribing period soon thereafter. I would check with Dr. Elaine LeVine (google for her e-mail address) about the specifics regarding psychopharm training and the prescribing law in NM.
Because the majority of folks pursuing RxP training have been in practice for years, little is known about how fresh graduates, who still have internship and other post-docs to complete, should go about pursuing RxP in states where RxP legislation has passed. There must be some way to complete all the training concurrent with internship/post-doc. I would imagine that some neuropsychology post-docs would want to also complete psychopharmacology training. It would be silly to have to complete the training separately.