The original outlet was just a 45 degree elbow. Yesterday I added another foot of pipe and an additional 45 elbow.
The pond finally drained enough to break the siphon. I still have a good amount of water coming in so later today I'll find out if the new fitting will be enough to prime it automatically.
Btw, it's 6 inch pipe, not 8 inch. Once I started the siphon properly it was enough to bring the level down 3 - 4 inches in an hour with fairly heavy flow coming into the pond. If I can get it to start itself I think the size of the pipe is fine.
This is with the new fitting:
