> The only data I care about on the phone already gets backed up by Google and is then exported regularly via Google Takeout.

Are you using Google Authenticator?

If so, how do you backup your secrets?

It has an export function that you can scan from an old phone. And I kept the setup keys in my keepass database.

On top of that now Google backs those up for you too.

Plenty of TOTP apps that sync (including google)

It does do that similarly to Authy now doesn't it? I saw a thread here recently with people arguing it broke MFA to do so.

I use a self-hosted Bitwarden instance for that kind of stuff and before that I used Aegis + Synchthing.

Google already automatically backs up secrets to your Google Account

aegis plus syngcthing fork works well

aegis authenticator uses google's android backup system.