izzy I'm also interested in the "several reasons" you've discounted OnlyKeys and SoloKeys as options. Any chance you can elaborate on that point?
SoloKeys only supports U2F and FIDO2, but they cost the same as the alternatives, which have a lot more features.
OnlyKeys doesn't have any external third-party security audits, and they allow you to backup your keys to a file. In my opinion, nothing should leave the security key, which is the case with YubiKeys, SoloKeys, and NitroKeys.