I’m a reverse engineer (mostly as a hobby) so I welcome the challenge should I need that at some point. :)

Although… it’s well known in information security to not roll your own cryptography or to rely on security by obscurity. And in my experience, most proprietary 2FA apps do use some form of standard 2FA algorithm, they might just be using a proprietary way of sharing the 2FA secret, have undocumented APIs for rolling secrets, or are using lesser-known (but still, standardized) 2FA algorithms like HOTP, which are all reasonably easy to figure out for a determined enough developer.