Best I can do is

"\ude41🙂".split("").reverse().join("")

returns "\ude42🙁"

On my machine at least man openssl shows that -k is for specifying the password you want to derive the key from, so in that case I think you are literally using the string /etc/ssl/private/etcBackup.key as the password. I think the flag you want is -kfile.

You can verify this by running the command in strace and seeing that there is no openat call for the file passed to -k.

Edit: metiulekm@sh.itjust.works beat me to it while I was writing out my answer :)