You want to take I-35W thru Denton, not I-35E thru Dallas. That is the easier route.
They have been completely rebuilding I-35 a section at a time thru Texas over the last number of years. Much of what was once rough 4 lane (2 each way) is now 6 and 8 lane all new highway. The current section beginning rebuild is thru the Waco area. Enjoy it before and after Waco, and suffer thru the Waco section. But they generally do a very good job of keeping traffic flowing. Been driving it several times a year for the last 51 years.
Austin and San Antonio can often be heavy traffic slowdown. You can bypass that by taking the toll road from just north of Georgetown down to I-10 east of San Antonio.
Time your travel to avoid large cities during rush hours.
Many people who do not like interstates take US 281 from Wichita Falls to San Antonio to bypass much of I-35, some 2 lane, some 4 lane, thru a lot of small towns.
The VERY large Winstar Casino on I-35 in Oklahoma just before the Texas border will be the last casino you see as Texas has none. But we do have great rest areas.