Keep in mind that in the future Haiku may have multiple users and each will have their own B_USER_DATA_DIRECTORY and B_USER_SETTINGS_DIRECTORY and others. Meanwhile the ones like B_SYSTEM_SETTINGS_DIRECTORY are global to the system, shared by all users. Similarly your app can be installed as a system app (the current default) or a per-user app (use pkgman install --home blah.hpkg). The search priority is for user specific apps first, and non-packaged before packaged within that.
But for your particular app, if your data isn’t changing much, you can include it in your .hpkg package, and use a path relative to the installed package location to find it (something like …/Data/MyApp/ off the initial current directory when your app starts). If your data is changing a lot (or has multiple languages or user contributed items) then put it in its own package and hunt for your data files as files in subdirectories in the usual 4 main directories - user-non-packaged, user, system-non-packaged, system.
See the https://www.haiku-os.org/docs/api/FindDirectory_8h.html documentation for details, and places where things could be.