Beautiful little creek. Some of my favorite memories as a kid were fishing in rocky creeks like that where you had to sneak up to every pool. However, the pools would frequently hold a surprising number of aggressive fish!

You might do some searches on Pond Boss about outlet designs or even just read some of the threads under the "Building a Dam" section. When I read sections of Pond Boss, I frequently find an answer to a question that I didn't even know to ask.

I believe (non-expert) that the current best material for a large diameter outlet is to use heavy-wall steel pipe and weld the connections from the drop outlet to the horizontal run. That stuff should last over 50 years (?) in many situations.

Another option that may be available for you, but that is not common for most ponds, is to design to frequently use your emergency spillway. Most spillways take some damage due to fast-moving water during a flood event. However, if you have rock outcrops available on one side of the dam, then you could probably create a very robust emergency spillway that was almost erosion proof. That way you might get by with a PVC siphon pipe for 90% of your rain events.

Definitely be advised that there is literally nothing worse that can happen at your pond than allowing moving water to go over the top of your dam. It WILL start cutting down, and can cut 20' in an hour. Consequently, much of the advice you will receive will be to make sure you avoid that catastrophe.

I am not surprised that the old dam leaked a little bit. It is difficult to seal a clay/earthen dam against a rock outcrop. Further, even if you achieve a perfect seal, there is a chance that the rock formation itself will leak once you impound water. The rocks can either have small pores or a few fractures or joints. Either system can pass water.

I think the "safest" way to design your pond is with a very large outlet pipe. However, I don't believe the neighbor's pond gets by with the small outlet pipe you showed without frequently having water go through his emergency spillway. (Unless he has a very small watershed above his pond.) If your spillway design could easily match his successful design, then that might be a possible alternative to save some money.

Building a large dam always takes LOTS of money. Saving a little money on your "insurance policy" (your flood control structures) is not a great idea, unless you are very confident in your less expensive solution.

Hope that gives you a few more ideas to compare to what you actually see with your eyeballs on the site.