alt Hacker NewsHey I'm in the middle of doing a very similar thing, though I didn't go quite as far as OP. I'm slowly deprecating my old RSA SSH keys and replacing them with a combination of Yubikey, Password Manager, and alternative SSH auth:
1. -sk keys on Yubikeys (with touch enabled) are my primary keys, used for signing and always installed on every host I touch/etc. I initially planned on using the PIV slots on my Yubi's to store the SSH keys instead of the built-in FIDO2 setup, but I hated having to manage multiple moving parts (like getting pscd running on all my clients). The only main benefit of using the PIV slots was being able to set the "cache" touch policy: the YK would cache a touch for up to 15 seconds so doing something like "signing multiple commits during a rebase" would only take one touch instead of num_commits. The side benefit would have been getting to re-use the same SSH key across primary and backup Yubis. The simplicity of the FIDO setup won out and I decided to treat getting annoyed at pressing my Yubikey a bunch of times as a sign I should be doing some sort of alternative setup like a dedicated key. Most prominently, I've now properly setup my Ansible configs with dedicated deploy keys kept as secrets instead of reusing my personal key.
2. I use 1Pass and Bitwarden (work/personal) to store a secondary SSH key that's only used for basic Git auth or other low-risk tasks. This SSH key is loaded from the Password manager directly into ssh-agent via stdin so it never touches my disk; I use fnox to make this backend agnostic.
3. Work uses Teleport, personal uses Tailscale so I rely on those SSH auth most of the time. Nothing much to say here except that Tailscale ssh is really convenient :) .
I've also setup PAM login and sudo on my machines, which brings me back to the old Sun Ray/Smartcard days. None of this is really to prevent security breaches/dedicated attackers or anything, though it certainly makes me harder to compromise. I just wanted to make absolutely sure I couldn't accidentally upload an SSH private key somewhere.
Running very similar setups. PAM handles FIDO / security key based auth on my local systems for root users and sudo commands, as well as LUKS decryption.
I have a handful of security keys, so I have a helper script to drop them into new hosts and another helper script on my systems to import keys straight to ssh-agent once detected.
Has worked very well and I don't think I'll ever go back to a "password manager as SSH agent" again, but it's probably the best alternative backup. I still have Bitwarden holding some backup keypairs for this reason, but haven't needed to use them in months.