The answer from your oncologist would be that 1. mutations of any significance will show up in your PSA (0.2 or above) and the PSMA PET/CT scan (4mm and above). He will tell something is suspicious if either one or both are growing. The technology does not show lesions smaller than 4mm but for practical purposes it is not a matter of urgency. 2. You have to rely on other regular tests and scans to detect any other cancer, such as ultrasound for the kidney, blood tests for liver etc. You are right, unfortunately there is not one test that covers all. But a good oncologist with substantial experience should be able to give you a report with confidence whether you are OK.
That said, even doctors get into trouble with other cancers because they fail to detect it early.