What service to restart after modifying /etc/sudoers for macOS to make changes active without reboot? What's the service that needs to be restarted after modifying /etc/sudoers for macOS in order to make changes active without a system reboot?
None.  No system process reads /etc/sudoers.  The command line binary sudo(1) reads /etc/sudoers.
Changes are immediate when they are saved.