Thanks to everyone that has responded so positively, sorryy I took so long to respond myself. Since my last post on this thread, I've built a small greenhouse to house a 300g hospital tank, overfiltered by a 55g sand & gravel filter. I also built a 1500g quarantine tank, to welcome new fish and protect my koi from KHV. I have a few "test fish" (ugly) in the QT to become exposed to any new incoming fish. Got a 55g s&g filter on that, just good enough to get by for now.
I filled both new tanks with pond water, then used a lot of the cycled media to start the filters off. For right now, all of my other fish are in the 300g Hospital tank, the biggest being only 10", I've been checking and no detectable ammonia, nitrites or nitrates. pH has been running a little high in all my ponds, not sure why yet, but I have added bags of peat to each filter to slowly bring it back to where I like it.
With the main pond that this thread is about, I wanted to implement some upgrades and fundamental design changes, so it is emptying now. I'll be adding a few features, like about 2 or 3000 more gallons, a wedgewire sieve, a 25W UV, two pumps and some TPRs. We'll see what else strikes me by the time I'm there.
I'm also amassing materials for a decent sized goldfish pond in my back yard, plus a stream and runoff carnivorous bog (flytraps, pitcher plants, sundews, etc.). Oh, and a pool for the kids, and finish up my greenhouse. I'll be adding a few more pictures, too.