Authy gets recommended often here but got turned off of them because they require a phone number to set up the app on iOS. There's no phone number requirement for TOTP implementations so I eventually found Duo Mobile. This was before they got bought by Cisco.
Great!
(Side note: Authy backups are encrypted client-side with the user's backup password. They're not unprotected on a third-party server; Authy has no ability to decrypt them. https://authy.com/blog/how-the-authy-two-factor-backups-work...)
iOS TOTP apps all suck, it's amazingly bad. I installed like ~15 different ones. After the fifth try, I just had to know if it was just my poor initial selection or a general problem.
Each and every iOS TOTP app has at least one crucial problem - requiring a subscription, mandatory sync to a proprietary cloud, having no export-import, not having a watch companion, being from an unknown/generic developer, no support for longer TOTP codes (worse, some display it truncated!) or they're simply very buggy.
I settled on Step Two because it was like all the others, but not an eyesore...
Is there a standard app developers can use to securely sync/backup to for self-hosters? Is there a 'nice' UX/flow to connect apps to s3-style storage (enabling folks to use AWS/DO/Backblaze/whatever?) or would that be too raw?
One would have to set a password that they then store in a password manager, that is then accessed using the same 2FA protected by the password. Plus a mandatory PIN, with the same caveats. Cyclical or duplicate authentication is simply not good design.