Multiple physical keys in multiple formats stored in multiple places. I've got a few security keys: one on a keyring, one in a locked file cabinet, one locked in a safe. Backup codes are printed on good quality acid-free paper with good toner and then are put in acid-free envelopes or laminated, stored in a safe at home and a trusted family member's house.
I'm far more concerned with an attacker from the internet or destruction from a fire or natural disaster than someone using my computer at my home who happens to have my username/password combination as well.
I have two. One stays with me, the other in a safe.
Which is legitimately a pain in the ass to register, manage, keep in sync, etc but I only use it for a few very important accounts--Google being one of them.
You should have emergency backup keys as well printed on real paper. I have a set stored with our important files, and another in my nightstand. Having my phone stolen would be a damnable inconvenience, since that is my second factor -- although not via SIM, but via Google's in-app authentication. But it wouldn't be fatal.
I use andOTP, which is compatible with Google Authenticator and actually allows backing up the private keys (one of the key missing features of Google Authenticator, though maybe they've fixed that).
I'm far more concerned with an attacker from the internet or destruction from a fire or natural disaster than someone using my computer at my home who happens to have my username/password combination as well.