Yes they are IP68 and they can handle alot. a dude tested its durability (youtube link)
your right they have no NFC but ive not had the use for it, As for smart card functions im not totally sure what your referring too
They do support fido2 resident credentials (username/password less login) if thats what you mean with passkey support and your right thats limited to 12 slots. for normal fido2 u2f logins where you have the username and password its unlimited.
you can also push SSH keys up to this device etc
I havent tried disabling the pin for FIDO2 so not sure if its possible
Another great future is you get 24 username and password slots.. (spread with 12 each over 2 userprofiles if you set it up) you could have one to log in to your computer. store your email in another slot etc. and those also supports normal totp. (assisted TOTP) as the device cant remember time but you can set up udev rules to push it when you plug into computer or go to https://apps.crp.to to set it on any device you want
As for securely storing the backup encryption passphrase. you only need to set it once on device initalisation and it could be a secure diceware password you store in your head..
or you could use a wordphrase you could hash something and use the hash output as the passphrase, use a 12 word crypto seedphrase you back up the same way as your crypto seeds.. world is the limit honestly.
the backups always are typed out encrypted so you can just type them out in your mail client and mail them to yourself if you want with no additional work