Lots of good info, I see vipers point lots of ponds have goldfish and koi … mine is the same. The fish are breeding so must love their home. My local lfs was telling me when he picks his fish up he sees a guy from London, who puts big prices on the bags the fish are in so his customer think they are special. Koi he sells for $10 this guy sells for $300. The point is they are only worth what you can sell them for. pond looks awsome4 can you keep orfe there?
Next year for the expansion why not add: lotus and elephant ear for sun.
And some of the following as tankmates:
False map turtle (bring in in winter, maps tend to be less likely to travel away), your green terrors, some small to medium African cichlids,oscars,Showa platies,and a few hogsuckers, and some sort of hardy schooling fish such as Mascara barbs
Hope the upgrade goes well, don't rush to move the fish like I did. Deep ponds wont stop temps falling (unless you build a large lake) it does help fish feel more secure, larger nolume also makes conditions more stable.
when you do upgrade your old pond would make a great plant filter, you could run the old pond into the new. Those fish are beautiful, do you move them in over the winter ?