best worldwide researchers are dr brunetto pisa cisanello hospital and dr lampertico ospedale maggiore milan, but it is very difficult to get an appointment with them if you are not italian because i think they both work for public healthcare only
you may also try dr colombatto of dr brunetto team in pisa (one of my doctors), he works both for public healthcare and private in the same cisanello hospital.it is also difficult to get appointment i will PM you his email
AFAIK German health insurence can cover costs in Italy. Of course it would be easier to find doc who can do vit d3 and pth check and combo therapy in Germany but it looks its hit and miss if even possible
if it just for vit d3 intact pth check it is very easy, you just test both and urine/serum calcium and go for the optimum ranges
unless germany is like the craziness in US where you need a doctor to test your blood for whatever you like.....in this case maybe moving to italy or poland according to the closest country, in italy when you pay yourself you can have anything, doctors are only needed when you want free heathcare covered tests/visits
but if you want a scientist with open mind covering from vit d to hbv treatments (also experimental ones) dr colombatto in italy is the best where to go, pisa is covered by ryanair so it is even cheap/easy to go there