I know you asked for GUI, but CLI tools are better at this job since it’s very easy to batch process the files using a combination of find and parallel.
Also, it’s probably a better idea to keep the files as they are and use a different audio player on your device. I don’t use iOS, so can’t suggest an app. VLC or mpv should work, but I personally prefer music apps with album, artist etc. support. For android, the app Gramophone is great. Another way would be to serve your files via a music server like navidrome and using a client like Tempo (again, it’s the android client I like, but surely there will be iOS alternatives).