Unlike systemd, launchctl doesn’t provide integration a syslog-like service. (I guess Apple expects you to use Console instead)
Apple’s unified logging is unreliable and in many tests I have had messages disappearing, even when manually testing with the syslog standard C library function. For example, events disappear for some users when viewing them through log stream.
If you want to redirect borgmatic’s standard out and standard error to a $PREFIX/var/log/borgmatic/{stdout,stderr}.log file, I recommend wrapping borgmatic in a helper script like the following, which I have put in $HOME/.local/bin/borgmatic_timestamp.sh:
Create a LaunchAgent in $HOME/Library/LaunchAgents/net.yourdomain.yourprogram.plist. Substitute any $USER string for your username:
This .plist file configures borgmatic to run at the beginning of every hour. If you want to run borgmatic immediately, you can kickstart it using