"Killed [12345]" message can happen for a variety of reasons and aren't limited to out-of-memory situations. BTCrecover makes use of multiple CPU threads, so it's possible that if one of them crashed, the entire program terminates and the shell displays this message.
That's why i mentioned that guide. My assumption about OOM are unlikely after OP mention using parameter which reduce RAM usage & have environment with very big RAM capacity.
If btcrecover made any log files, it's best to check them to see if this is the true reason for the crash or if it's something else.
I've used btcrecover before, but i didn't remember see log file created. Searching on it's repository with keyword "log" only mention btcrecover make log file to save possible password from blockchain.com file backup.