If the official API is restricted (...), the app parses the website or uses an internal API instead
This is fine to me. Tor frontend or not, you could not access anything if google services went down. I'm all for people putting content on other platforms, but for now for consuming content from youtube I'm ok with adopting this solution.
Now, if you want to go on with a project that massively mirrors youtube, feel free!
Maybe you'd be interested by this project by Jason Scott from the Internet archive foundation - but meant for research purposes: https://archive.org/details/mirrortube
the most favorites on the archive.org mirror are videos showing nudity 🙄