I've been waiting for an open source TTS model that was actually good enough to capture some of the subtleties of language and synthesize them in a natural-sounding way that makes sense. I think I finally found one that fits the requirements.
Model: https://huggingface.co/fishaudio/fish-speech-1.5
It uses an encoder rather than relying on phonemes, and generations sometimes vary because of that, but the amount of errors I've gotten are minimal, and the variations in the generation are all surprisingly natural in slightly different ways, which is very exciting.
Give it a spin if you are also looking for a TTS model that sounds good. It uses voice cloning, so find a good 10-20 second reference clip to have the generations use the same voice.
You are right. Their description of "SOTA Open Source TTS" caused me to assume it was open source, but it's clear that
So, it's "source available" and not released under a permissive licence.