I love bougainvillea! Even though you just started your bonsai journey, I can tell you have a great, artful eye for it! You picked a beautiful specimen and I love how it has the nostalgic value to it as well.
I started my journey some years ago and also looking for my lifelong green companion. I always get the soil mixture wrong, sometimes causing death to the tree, but I recently bought a specialized mix for bonsai trees and it looks just like your soil mix!
Good luck on your journey! Can't wait for more updates!