Ive heard rails are easier than boxes
You have HEARD rails are easier than boxes? Have you ever actually tried them?
A box is much more forgiving to learn on because it does have a wider surface area to you can work on centering yourself and staying still and practicing spins. The cavet is that you have to be going faster for a box because it is stickier will slow you down more than a rail will. But if you hit it with decent speed you will be fine. Speed is key in the park, being afraid to hit stuff with speed is the first thing you need to work on if that is an issue. After 2-3 years, with 10 times a year, if you have carving mechanics and good comfortable edge control down that i would say its safe to start mixing in park.
Some of the best practice for park is to first work on natural features, little jumps, rocks stumps and gap type shit to get you comfortable with added features, and learn to spin flat ground 180s. I would work in the morning on carving down blacks and getting comfortable with high speeds on those and practicing and focusing on balance edge control drills, then for the second half of the day go hurt yourself in the park. Start on mastering ride on boxes and rails and small jumps and grabs. Then once you can get on all of those features work on hop on boxes and rails and grabs on bigger jumps, then once you have a good feel for most or all of the types of features start throwing down tricks for each feature. It will be a lot of shit eating and a lot of falling, but when you can bag that trick you have been trying over and over it will be a momentous achievement