Just out of interest, what exactly do you want to see in the code? It's just a wrapper over OpenAI API, see https://platform.openai.com/docs/guides/text-to-speech + Google OAuth2 authorization + lemonsqueezy as payment platform.
I would love to contribute and run a version on my server. Also you need a search engine. Your list is too long to click through categories. The cool thing is you have monetized it -- virtually no open source projects have monetization built it.
If you don't want to open source it, send me an email: [email protected] -- i'd like to collaborate with you privately if I can run my own instance.
I'm really not sure about fully open-sourcing it. It's generally good for developer-focused products, but for Listenly... I just can't see the benefits. But I might be very wrong.