@provels said in mailreport - extra 'boot completed' time option?:

I already get a "bootup complete" email notification.

We all do.
That is, if the notification system has at least one of the smtp/telegram/whatever already working.

It's the last line of the all might /etc/rc.bootup that sends this notification message. It's this file that tells pfSense that it is pfSense and not some vanilla 'FreeBSD' device.