I don’t know about your computer but on mine there is no such thing as a fast hibernate. 
That means, in a very hypothetical situation, hibernate wouldn’t complete before the computer is snatched, and cold boot / liquid nitrogen attacks are possible.
Fast shutdown is probably safer for this application.
Presumably this is solvable by clearing the key before going into standby and then re-requesting the password to get the key back after coming out of standby. That is a convenience v. security trade-off.
That approach would presumably also work for lock but there is a big ‘if’ with that because, while the computer is locked, many processes operate in the background and some of them will need access to the disk. So it is almost as if “lock” has to become integrated with “standby”.
1 is probably the hardest case. Solution may be use the script at https://docs.puri.sm/Librem_Key/Getting_Started/User_Manual.html#automatically-lock-the-desktop-when-removing-the-librem-key so that the computer locks when the key is removed and have the discipline to remove the key when going to bathroom, getting a (fake) delivery, etc. and solve the problem so that security is enforced when locking, as per the previous paragraph.