Hi, I'm not trying to be a party pooper but, what if (a question you should always ask in this hobby) the corona thing doesn't end in summer?
What if out of nowhere it blows up and gets 10x worse?
Or what if your parents change their minds and dont want a pond anymore come the summer?
What if the world economy is so bad at the end of this that you can't afford/source what you need to build the pond?
What if the local council won't let you build a pond?
Hypothetically if one of these scenarios were to happen and you already had monster fish in a grow out that they were quickly outgrowing, what would you do? What happens to the fish, will you euthanize them?
Will you give them away? What if no one wants them?
Will you keep them in a tank that is too small for them until it all gets better? What if that takes months, or even years?
Basically, you don't yet have the fish. You have the power to not get the fish until you've built the pond.
I know its hard, you want them and have the best intentions to upgrade their home as soon as possible, but as a lot of adults will tell you, life doesn't always go as planned unfortunately.
An experienced keeper can get away with an impulse buy every now and then, someone with little experience should try to avoid buying fish that they can't yet house for life.
Tl;dr?
You don't have a pond yet so you have no need to buy monster fish until you have a place to house them at full size.
It would be safer and more practical to have a home for the animal before acquiring it.
Sorry if that comes across as rude, I don't mean for it to. Just some friendly advice from someone who knows that life doesn't always go as expected.