I didn't realize that you had already committed a systemd unit: https://salsa.debian.org/wouter/ola/-/commit/70f1e074866c246a8cab9c7b785e453112b69acc
Can I ask that you remove the --syslog parameter, or move it to /etc/default/ola instead? If it's hardcoded in the unit file, there's no way to override it without replacing the unit. I really don't think it makes sense to retain syslog when journal logging is available.
