On reflection.. has anyone else requested this in recent years? It seems unfortunate that the behavior we had pre-discobot is now changed so it requires discobot to be enabled to send a welcome message, and that it must come from the discobot account.
IIRC, it used to be these messages would come from the system user or the username specified in the site contact username admin setting.
I wouldn’t advocate any change here if we don’t have rule of three, of course. Folks seem to like the robot. 
And of course not everyone has the automation plugin, so relying on that to send welcome messages is also suboptimal.