Cannot cancel my voluntary health insurance

I don't see your problem at all. One lives in Germany and is insured here - or they don't live here and aren't insured here. If you leave then you give up your residency and it makes no sense to be insured any longer here. Completely logical. But as long as you officially live in Germany then of course you have to be insured like everyone else. The basic rules don't change just because you seem to be indecisive. Paying insurance would thus be the cost of your delaying to officially give up your residency. But this is YOUR choice – and you have to decide if it is worth it for you to try to leave such an option open. And maybe it makes sense for a month or two to see if the situation is really as expected. But then you have to decide. One cannot legally and practically live long in another country and still claim residency here anyway because it will quickly be a complicated tax situation in addition to the health care one.