Congrats on the small child
someone you can train shortly to reach the hard to get at bits in the back of the engine bay!
As for your car- sounds like you're still continuing to see good progress in your consumption- I think that Arragonis may be right- my best tanks in the iS happened when I had 3 people plus luggage in the car on the motorway.
Thinking about your need for a smapll car, would it be possible for you to book a hire car on the days that your wife has appointments?
That way you'd be paying a little, but avoiding the costs of actually owning the car?