I mean if I sent a post on Damus, and a toast popped up after a few seconds saying "Sent to 3/5 relays", to me that would be meaningless if Damus kept trying to send to the other two relays.

It wouldn't help with confidence at all, because maybe a short while after the other two relays will take it. Who knows?

For such a notification, it only makes sense to me if it's an end result and not an update at a point in time.

Like on WhatsApp, when you get a message failed notice it means the message failed and WhatsApp is not going to keep trying anymore. If you want to try again, go ahead, it's on you.