I know there are several ways to send messages to devices in TTS format, but none of them seem to support the combination Piper and the official Home Assistant Android app. This lack of functionality ruins the user experience.
so TTS on the device uses the default TTS engine which is typically Google or the manufacturer. For Piper all you need to do is install the correct TTS engine then in your device TTS settings change the default engine. The companion app will then begin to use it. Looking online it seems the engine already exists https://github.com/rhasspy/piper/issues/103#issuecomment-2333150491
I “fixed” this with installing “Music Player Daemon (MPD)” on all this devices. It works well with TTS piper.