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 :)
It is likely not worth your effort as whatever you come up with will likely result in discord deactivating your account for breaking their ToS, or them breaking their API forcing you to constantly play catch-up.
This is why open communication protocols are so important. Email is still as ubiquitous as it is because it's a protocol, not an API.
I personally think it would be less overall effort to get your friends to switch to an open protocol like matrix, or XMPP than it would playing cat and mouse with proprietary APIs. But you do you, I wish you the best of luck!