This is perhaps why FIDO2 works best when combined with single-sign-on systems, such as those promoted by large email providers, etc. Fewer accounts to have to manage 2FA devices for, and a greater chance that you've already signed in and authenticated your devices with all of them.
Personally, though, I use a password manager, and have some (but not all) sites tagged as 2FA in the password manager. So if and when it's time to add another key, I can just go down the list. Not as convenient as SSO-based 2FA, but sometimes you really don't want to sign in with Facebook, say. :)
and how do you do estate planning? I'd like to give my family access to all of my private keys for everything when I pass.
Register multiple/duplicate keys.
As for estate planning, set up a spare key that you can keep at a relative’s place, or add others’ accounts to your “Family” in Google/Microsoft/Apple/etc. Either they have their own keys and the company is aware of the handover or they have a copy of yours — such as you logging in on their device or keeping a FIDO2 key at their house and they can pretend to be you. A service like 1Password Family could also be of use here.
Just like any other password or data, 2fa strings also need to be backed up, like in a password database (separate from the usual one).
Unfortunately most people have only one phone, so that didn’t work until options came along where you could add more than one token/device instead as backup.
If you have that seed phrase, & any device with correct time can calculate the TOTP code, even a simple local javascript app.
Obviously that phrase leaked would mean hacker can also generate codes. So that's why those phrases should be kept extra safe, away from normal passwords.