I store passkeys and totps in 1Password. I know it means there's no hardware protection of the secure element, but in return they're trivially synced across my devices.

I feel this tradeoff is worth it to me; certainly it is no worse than email or SMS as the second factor.

Chrome Sync, iCloud Sync. There are great answers for this.

Sure. But if you sync passkeys, are there any advantages apart from phishing protection?

The biggest advantage for me is using the hardware secure enclave, thus effectively getting a 2nd factor.