I'm sure this won't be a popular opinion here on MFK...but even a 300-gallon tank is woefully inadequate for a RTC more than a year or two old. There, I said it. Back in the day I had that exact fish in a 360 and it did not take very long for the fish to take on a very sad, pathetic aspect as it shuffled around in that relatively small area. Bear in mind that you can practically see those guys growing. I don't use the term "happy" to describe fish...but if I did, that fish was not happy. Neither was I.
Sure, the pond is a great idea. I had the very same plan when I was in my 20's, along with a lot of other grandiose schemes. Some of them came to fruition; a lot didn't. This pond sounds good on paper and in theory. It's the cost, the execution and the required space that are the sticking points.
Buck the trend. Break the mold. Instead of being just another keeper with a too-big fish in a too-small tank...buy/build/set-up that forever home for it before you get the fish. Buying a tiny redtail now and sticking it into a 50, or a 300, is like pulling the pin on a hand-grenade before you quite figure out where you are going to throw the dang thing; it very quickly develops into an "uh-oh...now what?" moment.
Edited to add: What Esox said! Jeez...everybody types faster than I do...