If you set mary TTS as the TTS for rhasspy you can use the http api for for TTS.
It is IP:PORT/api/text-to-speech?siteId=SITEID just post text to it and it shoud say it. Because i do not use homeassistant i have no idea how you can do a http post from homeassistant.
for more information on Rhasspys http api:
I used MaryTTS but the german voices did not really sound better then picoTTS which included things like pronouncing dates and some units like °C correctly.
I use node-red for my intent handling and sending the text to speak to rhasspy.