Here's an idea: Why not allow the owner of a Google account to delegate a trusted third party who can handle MFA/otherwise approve logins on their behalf. I kind of do this already by setting the recovery emails for family members (especially aged parents) Google accounts to those that I control, but to my knowledge it is not possible to do the same for the mobile number used to secure the account. This way, at least…
So the biggest problem is that every optional feature you offer up does not help when someone walks into the library already locked out. The vast majority of society will never be aware of available options and features for their Google account, so it's only the default behavior that matters. Most people locked out of their account could've set up some sort of way to get in (like backup codes), if hindsight was 20/20…
Fair enough. I live in the EU, we all have state-issued ID cards and no cultural problem with using them, so presenting those via some channel to Google would work here, but I can't imagine it working in the states.