The most economical/reliable 6-seater is probably going to be a Mazda 5. It's 6 seats in 3 rows, but kind of midway in size between your Yaris and a conventional minivan. They're relatively cheap used. Beyond that, you're probably looking at a conventional minivan like a Toyota Sienna or Honda Odyssey. If you can make 5 seats work, a lot more options open up.