I think you should make all the TTS solutions plugin solutions for a overall Home Assistant TTS solution.
This way one could easily swap out one TTS solution with another, improved one.
The plugin would hold the settings for the tts, eg. if it’s google_cloud_say, almond or what ever, and then just plug in to the homeassistant.tts_say or something like that.
It’s nice that the cloud_say has these settings in a central location, so why not move it a further step and make that work for all the tts solutions that you support. I think it would be easier to maintain to have that abstraction layer in between.