Well, to give this closure, rsgain definitely did the job of adding Replay Gain tags to the tracks as promised, but either the player on my phone doesn’t use them, or for some other reason the results were not as satisfying to me as with normalize-audio. I ended up writing a script which does exactly what I want, using ffmpeg (to convert the few tracks that I was only able to buy as mp3s to FLAC), metaflac to save off the tags I care about, normalize-audio, and then metaflac again to restore the tags. For convenience in my use case I also have it tag all the files with the directory name as the album title. So essentially, I am creating my own compilation albums.