I haven't re-evaluated OSS TTS options for a few months but from my own experience earlier in the year I've been pleased with the results I've gotten from Piper:
* https://github.com/rhasspy/piper
I've primarily used it with the LibriTTS-based voices due to their license but if it's for personal local use you can probably use some of the other even higher quality voices.
The official samples are here: https://rhasspy.github.io/piper-samples/
Here's a small number of pre-rendered samples I've used that were generated from a WIP Piper port of my Dialogue Tool[0] project: https://rancidbacon.gitlab.io/piper-tts-demos/
While it's not perfect & output quality varies for a number of reasons, I've been using it because it's MIT licensed & there's multiple diverse voice options with licenses that suit my purposes.
(Piper and its predecessors Larynx & Mimic3 are significantly ahead of where other FLOSS options had been up until their existence in terms of quality.)
[0] https://rancidbacon.itch.io/dialogue-tool-for-larynx-text-to...
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Edit to add links to some of my notes related to FLOSS TTS, in case they're of interest:
* https://gitlab.com/RancidBacon/notes_public/-/blob/main/note...
* https://gitlab.com/RancidBacon/notes_public/-/blob/main/note...
* https://gitlab.com/RancidBacon/notes_public/-/blob/main/note...
Quality seems fairly similar to Coqui-TTS, though maybe the intonation is a bit off.
I've generated a bunch of samples for comparison here: https://gitlab.com/danhawkes/piper-tts-test
A former Mycroft dev, Michael Hansen[1], is still building several year-of-the-voice projects after he was let go. I'm especially excited about Piper[2], which is a C++/py alternative to Mimic3.
[1] https://github.com/synesthesiam [2] https://github.com/rhasspy/piper