I used an app called Noice from F-Droid which has a lot of pleasant background noises (including white noises which I sometimes use while sleeping). One update, the author suddenly added internet permission, pulled out all the noise music files from the app and put them behind a server. Now it only works if you have an active internet connection OR you buy a premium.
Thankfully, I still have the last “good” version.
Have you tried fcitx with the Anthy plugin? I use that for Japanese.
https://github.com/fcitx5-android/fcitx5-android