You could just use my addon, which handles the Greek, and other eastern European scripts, automatically:
https://revad.github.io/mudcat_tools.html
(Unless you use Safari.)

I suppose it was more 'Tha-ees' in the original Greek: Θαΐς
Wikipedia

I just listened to one of the Massinet arias "Thaïs, soeur des Karites" and ... it's not clear.