I'm vaguely familiar with ram pumps. I don't know the answer to your question but I have questions that maybe will help?
1) Would there be an efficiency loss in pressure that prevents the ram pump outlet pipe from reaching the same height as the drive pipe? My vague understanding is that ram pumps work better when there is head pressure generated by an elevation differential or a strong current. So maybe a 10 foot drop in elevation buys you 8 feet of output. (Just making those numbers up.)
2) Would the waste valve slowly drain the pond over time?
I realize that those questions are partially what you are asking answers for but it is where I would apply my research.
"You can't get something for nothing." A ram pump depends on water flowing downhill to pump part of the flow higher than the source. The rest of the water is waste and there is no way to get it back without pumping more water into the system. It cannot circulate the water in a pond while keeping all the water.