Forms are very useful for data entry on small displays on the iPhone and iPad. As you noted, they are very simple, not customizable.
Forms aren't really needed for data entry on bigger screens, and are not a feature of Numbers for Mac.
If you need structured ways to enter data then perhaps you might need a database app rather than a spreadsheet app.
Otherwise take advantage of the bigger screen to enter data wherever your document design indicates. I tend to try to keep things simple. The built-in templates at File > New in your menu are great examples of how to structure documents in Numbers.
SG