I just want to be clear on your budget because you started with an $8K ask that has now more than doubled based on expected maintenance and all the paperwork associated with buying the car.
Based on a budget of $20K, however, I don't understand why you haven't been testing a Passat TDI. Last time I was looking with purpose, which was at the beginning of this year, CPOs with less than 20K miles were going for $18-20K. With taxes you'd be either hitting right at or just barely going over your budget, but with damn near a brand new car with a few hundred thousand mile warranty (IIRC, CPO has an even longer warranty).
Have you tried a 2015 GSW or Golf? I'm trying to remember the specifics but I think the newer platform has more adult legroom at the expense of the cargo area (the overall dimensions are essentially the same). That said, my 6' 7" buddy almost passed on the Golf I got him into for the Passat because there wasn't any legroom for passengers with the seat all the way back. He ultimately chose the Golf after I asked him if he was buying the car for him and his dog or for his buddies and family members he might take to dinner and a movie one day, but I suspect it'd be a tight fit for a carseat. I can check in a day or so if you're at all interested.
I'm 5' 11" and the rear facing seats in our GSW are a tight fit. Not uncomfortably so, but questionable if I had another 6" somewhere. In the bug I had to sit straight up or maybe a little leaning toward the steering wheel
The Passat is a larger class of car than the Golf. If you haven't compared them back to back, the difference is obvious between the compact and mid-sized.