score:6
Parkopedia has plenty of parking spots marked out around St. Pauli. With costs ranging from 0 to 37 EUR per 24h. Some are hotel parking spots, with reduced rates for their guests, who nevertheless allow other to park too.
Arguably the highest level of safety will be provided by an underground / locked parking with CCTV, as opposed to a public street parking. However those parkings are unlikely to be cheap. Up to you to pick a place.