Generally speaking, Chinese universities hire foreign teachers on ten- to 12-month contracts that commence in the fall semester.
When universities hire a foreign teacher for the spring semester, it is usually to replace an incumbent teacher who will be leaving prematurely for one reason or another. Perhaps conditions are unbearable or, through no fault of the university's, the teacher has to return home to attend to an emergency.
Because most former applicants are no longer available—and as time is of the essence—the FAOs will typically be forced to hire anyone they can get at a moment's notice. As foreign teachers in their sixties tend to overrepresent those who still happen to be available for employment in the spring, it might be true that they have a better chance of being hired during mid-year, as do all others who were not competitive for fall semester employment.
Unfortunately, this is not something you can reliably count on and I would not advise it as a job-seeking strategy because, first, you could very likely find yourself without any position come next spring and, second, it greatly increases the likelihood of working for an undesirable school, i.e., one that cannot hold onto its foreign teachers for a full year.
Your best strategy, as you indicated above, is to focus your job search on less desirable regions and lower-ranking universities, i.e., areas and schools that cannot attract the more competitive teachers. This will also provide you with a greater opportunity for negotiation as well. I agree that a workload of 20 to 22 periods per week is excessive, if not abusive, and you should be able to find a less demanding position if you start looking now.
Best of luck to you and let us know what happens.