Nice! If I understand correctly this is fully local and doesn't depend on cloud stuff? After a few failures I don't want anything that relies on somebody's buggy cloud systems.
Neither are required. You can happily open up DMZ and use whatever TTS engine, yourself.
I have HA up and running but have only used its web interface so far.
Some are a perpetual cat and mouse game though for the companies that simply must snoop your data as part of their biz model or have another reason to be terrified of you directly talking to their things.
And like any smart thing you put in your house, some things you need to block at the DNS level if they are hardwired to phone home constantly. But yes, Home Assistant and choosing the right things makes it possible to keep everything internal for sure