What’s the missing piece compared to Authenticator Plus or Authy? What’d be my actual missing piece would be backups to my own infra, rather than iCloud or any other third party.
I think what is killing adoption here is the reliance of so many third-parties on Google Authenticator style OTPs. OTPs are terrible -- you completely lose 2FA if you lose your phone, and OTPs are easily phished. I don't know why it's so popular, whether it's some regulation, or whether people just wrote their 2FA code a decade ago and haven't updated their flow to accommodate newer standards. Whatever the reason, it is just driving people to develop and use cloud-based password managers and OTP syncers like this, and those things aren't helping security. They're giving people a false sense of security; they feel like they are as protected as having a physical token required for all logins to important apps, but without any of the protection. It's less than ideal.
OTP is bad, as you pointed out, but backing up the seed does not make it worse.
I admit I use Aegis because of the backup functionality. (Well, also for the support for Steam...) After using Google Authenticator, losing my phone and having to use backup codes and painfully re-enable 2FA on all my accounts, I realized that using an authenticator with backup is much simpler than the backup codes.
Things would be way better if people just implemented WebAuthn, though. It's more secure than OTPs, and it's so transparent and easy to use that people think it's less secure. Passwords and OTPs should go away, and it's only a few lines of Javascript to make them go away. Sigh!
I was desperate for an app like this a couple years ago, but 1Password wound up adding mfa. Regardless, I wish the creators luck, this looks very good! A real “mac-assed app” https://daringfireball.net/linked/2020/03/20/mac-assed-mac-a...
Edit: OTP Auth has a macOS version, too. Separate purchase.