@ exactly 7AM on the day you plan to go, get in line online here: https://www.dps.texas.gov/driverlicense/getinline.aspx?location=https://getinline.dps.texas.gov/wa-services/txdpsservices/waweb/default.aspx?siteid=TXDPS123 ... if you don't register at exactly 7AM, you'll miss your chance until the next day.
Unfortunately there really is no best time unless you camp out at 5 am
I suggest you go to the Texas Department of Public Safety Driver License Mega Center in Carrollton. You will avoid a long wait.
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