If you ask me there are 2 ways how to make this project safely:
1. Use of heavy helicopter.
2. Extend the runway at least twice or more, this will make it suitable for AN32 and some other aircraft.
Neither of them are cheap. But flying 4-6-8 times a day to 500m at 5000ft at the edge of any aircraft performance means that it is no longer a question IF accident will happen. It is a question WHEN. And I bet it will happen pretty soon. Try to count crashed freighters around Jayapura and Wamena, which are far less demanding than your place.