Having played around with flite a decade ago, and at that time feeling that it was already then nowhere close to the fidelity of other speech synthesis examples. I find it surprising that there still isn't anything better than festival/flite? It sounded then like a clunky robot, and still does today. Surely some of the many research projects have released their work as open source?
Work like, say https://arxiv.org/abs/1806.04558 [paper]
https://github.com/CorentinJ/Real-Time-Voice-Cloning [repo]