Sarah, I say go with your gut instinct. If you feel that your baby would benefit from not being born until at least 40 weeks then go with that. Based on your past history I don't see why they would not agree with letting you book an elective cs a little later than the standard 38-39 weeks.
Im sorry you did not get your VBAC with Juliette and if you do decide to have an elective cs with #3 you know there are things you can request to help make it more special and less traumatic for both you and your baby so putting into place your "ideal" caesarean birth plan may help you to make peace with deciding to have an elective cs next time?

Oh, and back to the original question: my elective cs was at 38+1 weeks but only because I had PE and OB's wanted her out. She was born absolutely fine with no breathing problems too BTW.