The most user friendly way for someone with only one device and no external trusted screens is with the help of a recovery enclave all your secrets are encrypted to.

Whenever you add a new secret, you have it encrypt a copy to ideally a smartcard on your keychain and as a hail mary to each of a quorum of of keys held by secure enclaves that run an open source remotely attestable VM using TDX or SEV-SNP for provably encrypted memory, proof it is not logging, etc. You can also shamir-split encrypt to m-of-n geodistributed key shares stored offline as a double hail mary. All of this would be automatic and transparent to the user.

When you add a new device you must approve from an existing device.

That cryptographic approval, ideally a passkey tap, can start a process to bulk decrypt passwords and re-encrypt them to the TPM key in the new device in a hosted remotely attestable secure enclave the device verifies. The user can be given a chance to cancel this transfer within a reasonable period of time like 48 hours. This extra time could be bypassed if a user has a second device attached to their account. A user could also release one credential at a time manually and explicitly on both devices following the usual rate limit rules etc, or bypass the limit if both devices can be directly connected, or use an offline recovery yubikey or last resort offline bootable recovery usb for expedited recovery as well, etc. Good to have multiple fallback recovery methods that tolerate at least one machine being compromised at all times.

This is just one example scheme. There are many many ways to solve this in a way users are protected without them having to learn to do anything more complicated than account recovery on any web service ever.

I really hope someone rips this off and runs with it. My team and I have open sourced everything required, as have many other teams.

You use a yubikey? Duh?