Spitballing... maybe rotations are by subject area, but you'd have to have passed qualifying exams for certain posts within that subject. People who didn't find the more responsible posts to their temperament would still be exposed to the problems of the field as they cycle through, and those who do would be doing on-the-job training until they felt confident in (or the other people currently in that subject strongly recommended to) challenge the exams.
(come to think of it, that's not too far off from the apprenticeship model of many trades and professions in my country, except it would tend to develop T or even W shaped people instead of just I shaped...)
Another thing to consider is that, at least in my country, police have a huge mandate. If we narrow the scope of the mission, we can reduce the amount of training necessary.