If you’re at a point where you would have your users pay for a 2FA app why don’t build your own (or build TOTP functionality into some existing app)? TOTP is very easy to implement yourself, it’s literally just a handful lines of code.
It's not that it's hard building it, but: Do you want to maintain it? On iOS, on Android? Through all update cycles? On all screen resolutions? Keeping up with the regular bullshit, especially coming from Apple? Random app store bans? Reviews? Support? etc.
