I suspect that any of the methods proposed here would be prone to a C&D, but IMO the safest legally would probably be the RSS method (not a lawyer though). Reddit’s RSS feeds are public, documented, and available without the need for private APIs, authentication, or an API key, so I don’t see how they could claim that a wrapper is unauthorised/illegal. Documenting their private API however seems like a gray area. Google LLC v. Oracle America, Inc. found that APIs are copyrightable, but this use may constitute fair use.
Making a breaking change to the mobile API also breaks old outdated installations of the app. Websites and their APIs are usually synced, apps not so.
If they were really motivated to stop your method, they could just obfuscate the frontend with webpack and break your scraper every time they make an update.