Is there any way to encrypt my drive with a private key, and no external tools (to be installed), and then unlock via command-line?
I'm referring to my second drive and not my boot drive. I’m not keen on turning on file-fault.
You can use APFS encryption (essentially FileVault) on a secondary drives such as USB drives etc. When formatting the drive with Disk Utility choose “APFS (Encrypted)”, and you’ll be asked for a password for the drive.
When you plug in the secondary drive, macOS will automatically try to mount to mount it by opening a pop up asking you to enter the password to unlock.
If you literally want to use a private key, as in a key file (i.e. not a written password, but an encryption key) - then you’re out of luck. No such solution comes built in with macOS. However it can be done with external tools.