A Yubikey for a phone or an iPad seems pretty clunky. Outside of super high risk people and on-call devices, are there people actually using physical keys for their mobile devices ?
I actually do and it works pretty well. My Yubikeys are NFC compatible. I have my OTP generated on it and also use it for U2F / Webauth. I only need to tap my key to my phone for 2FAuthentification.
Or just get some Yubikeys.