There is a reason they don’t send it until someone is online. On iMessage, you know if someone read it, not if they actually are able to receive it. If they fix the bug where the time of the message is when it finishes sending, it will be a great feature because you know if they have access to their phone and data. It will try to send it throughout the down time. Also you can use other rcs apps and have things go through rcs messages because of desktop authentication.