Hey Fatalitu,
I wouldn't recommend writing the JSON data manually to create a 64x64 version of the structure, since that can get very tedious, as you've probably noticed. Instead, here's a method that I think will be much easier and faster:
Open a Superflat world in Creative Mode.
Build your 16x16 grid template on the ground.
Place multiple copies of this template next to each other until you have a 64x64 grid. You can either build them all with AutoBuild or use commands to copy and paste the blocks directly.
Disable AutoBuild and enable TemplateTool. Use it to scan the entire 64x64 grid structure.
You now have a 64x64 version of the template that you can use with the AutoBuild hack.
Hope this helps. Good luck with your cactus farm! 🌵