I-35 is the primary N/S road through central TX. And I would not recommend it as it carries heavy traffic that bogs down regularly. Knowledgeable travelers stay off of I-35 and use parallel roads on either side of it. In the case of the OP, I'd recommend 287 S/E out of Amarillo to Wichita Falls then take 281 S to Lampasas, taking 183 into the Austin area. I highly recommend stopping for BBQ in Stephenville at the Hard Eight BBQ joint. It is right on the highway.
Thanks—this is the info I was looking for....didn’t really want to have to drive I 35 if I could avoid it. And I know there are plenty of great secondary roads in Texas. Have been on many of them just not in this area.