Do any of them work on desktops? I keep around a spare iPad to run my authentication apps, but I'd rather have it installed on my computer instead.

Authy does but has some issues showing same site names as on PC so not perfect

1Password has built-in TOTP support, though it's a little overkill if you only use it for that purpose.

On Windows there's WinAuth: https://github.com/winauth/winauth

It doesn't seem to be updated anymore, but it works well.

Then use other ones :)

I currently use Aegis and Bitwarden. AndOTP also allows you to export tokens.

Google Authenticator now has an export and import feature where it bundles all your accounts into a QR code to scan on your new phone.

Might not be ideal for backup however