I would be quite interested in your app.
I teach English to high school students in China who will be going to universities abroad. Many of my students are weak with English listening, and I make extensive use of online videos, for which I get AI to create a transcript, and then blank out words to do listening practice. Finding the right accent I want, which also matches the topic and English level I want, can sometimes be very challenging if not impossible.
For my super weak students in the IFY (International Foundation Year) curriculum, I've pivoted towards British speakers/accent because their listening exam is with British speakers.
For my IBDP (International Baccalaureate Diploma Programme) students, they can have listening exams with a variety of English accents.
I have tried using Text-to-Speech on my Mac (running macOS 15.7.3 Sequoia), but the results leaves something to be desired, even with the enhanced voices.
I watched your Youtube video and the voice "Tess - British Female (GB)" sounds great for my IFY students. The voice "Zara - South African Female (ZA)" and would be useful for my IBDP students. I think it's much easier to do TTS with a voice/accent I select than the other way around, which is the way I have been doing it. I hope there are also Australian, French, and Indian accents as well for English.