Awesome looking tank.
I have some experience with Kerdi. They have a drain kit for showers which takes 2" PVC (I think, it's been a couple of years). Kerdi is an awesome material for showers, I never thought of using it for a pond. So if someone else is considering Kerdi, but their drain kit for bulkheads (although it is expensive).
This is one of the nicest looking DIY tanks I've seen. Totally blew me away.