We take I-35 from Dallas to Georgetown and back 2-3 times every year. With a few exceptions it has become a very good highway. Most has been completely rebuilt with an extra lane or two added each direction. There is still construction from Temple north for 20 miles or so. But most of that section now is on new pavement with some lane shifts. The only other construction area of concern is on I-35E from just south of Waxahatchie north for 10 miles or so. But other than lanes narrow in places it is not a real problem. I would not hesitate to tow our 5er there.
If you do take I-35, take I-20 east from I-35E, then I-635 north to I-30. That takes you around the worst part of Dallas. And do it mid day or weekend.
Many prefer secondary highways. But if I am trying to get from point A to point B quickly I take the interstates.