I spent this day doing the same!
I also tried 4-bit ADPCM, was able to get super small .wav files but unfortunately, it didn't play correctly on Fusion or iOS devices. :/
I'm using different sample rates for different audio. For example, some of my audio doesn't have high frequencies, so I go as small as 6,000 kHz. When I need "sharper" audio, I use 16,000 kHz. I'm using 16bit as 8bit has too much noise.
It's a bit silly to play around like this, but I think I'm going to get an okay sounding result. The size will be around 2½ bigger than on Android, but I just gotta live with that I suppose.