Good point. I guess instead there could be a “retry interrupted backups interval” field. If greater than 0 (default / disabled) AND an interrupted backup exists then at startup and every “interval” minutes an “is destination available” check could be done. When it becomes available is when the prompt could appear.
Of course if one has an unstable connection that could cause many annoying prompts so maybe no prompt is needed. The act of setting the interval could be considered “approval” to continue when possible.
I think I’d also assume incomplete backups take precedence over scheduled ones. So if an incomplete exists then a normally scheduled start of a fresh backup would instead just continue the incomplete one.