So you only edit the land inside, only then with a bit more margin from the edge.
Exactly. The park boundaries are really pretty big. Do you really need to build a theme park that big? Make the park just a bit smaller, and smooth the edges yourself. Not that big a deal. Or...
You already mentioned that you feel that you may not be the best park builder, so all the more reason to build small. Instead of thinking about building, just for instance, the entire Disney Florida complex, just focus on the Magic Kingdom, or just focus on Fantasyland and that's it. Make the best one you can, put it on workshop if you want, learn from your mistakes, then build a slightly larger park, etc. Eventually, you'll feel the confidence to fill up that map.
Finally, I think just about everyone has requested something from Frontier, and have not gotten their wishes either.