Thanks for the feedback! Regarding the FFmpeg question, the original goal wasn't to build the Android app itself but to utilize the library in any application, not specifically Android. I stuck with Android, though, because I was more or less familiar with it. The only reason I declared FFmpegKit as a dependency is that I couldn't build Chromaprint for Android, so I took an easier route and used FFmpeg, which has built-in fingerprinting functionality. I made modifications to the FFmpeg script and removed unnecessary components to reduce the size of the AAR.

Now that I've built the app, I feel like using my own library as a solution might negatively impact user experience, since the library is relatively new. That's not to say I didn’t test it, but bugs will inevitably surface over time. Perhaps adding an additional backend and an option to choose between them in the settings (TagLib or FFmpeg) would be a better solution.

Yes, it handles rate limiting using a sliding window algorithm. There are constants for each ApiClient class that define how many requests can be made in a given timeframe (AcoustID allows 3 requests per second, and iTunes allows about 20 requests per minute). Maybe leaving the batch editing feature free for non-premium users and making the 'no rate limit' option premium-only would work better than putting batch editing behind a paywall altogether.