I was in your shoes a year ago - with a Q5, a summer baby on the way, and a car seat that wouldn’t fit behind me.
People are right: for the first 2-3 months, you won’t be driving much at all with a kid. For the few pediatrician visits early on, you can make it work, even if it’s a bit tight in A5, while continuing to pay it off.
Before that, take a look at the Q7 or Infiniti QX80. It’s a 3-row SUV, and when the third row is folded down, it gives you a much larger trunk and noticeably more legroom in the second row. That gives you flexibility - whether placing the car seat behind the passenger or behind you, while your partner sits in the other side.
Get comfortable with the options and timing. You can revisit the decision when your baby is around 1-2 months old.
My honest advice: buy your child car seat first, then go to dealerships and test cars with the seat installed. Find what fits both your family and your comfort, while you have(!) a time now. Make your shortlist. Revisit them for the best deal when baby is 1-2 months old and A5 nearly paid off. Sell your Q5 to Carvana/Carmax/Private, and have a clean deal on new car.