This killer was driving around and stalking the victim's home that morning at 2am. Are they getting a sense of the area, the escape routes, etc? Does it prove they are not familiar with the location? In fact, they drove into a culde sac following the shooting and had to improvise to get out of it without coming back the same way. This could mean a stranger, or at least someone who had not been there before.
Also, moments before Sergio leaves for work, the killer is at nearby Goodard school as if he is waiting for husband to leave for work. 3 min after hubby leaves, killer pulls up to house. The timing is hard to ignore. Who knew Sergio's schedule, what time he would leave for work? Did he always leave the same time every day, or work that same day of the week.
From body language and supposed conversation, Liz did not know the person that shot her. She didn't greet him/her like she was saying Hi to someone she recognized.
I own a Nissan Titan PRO4x, the bigger model but same trim level of killer's truck. It is far less common and more pricey then comparable Chevy, Ford, etc trucks. For that reason, it seems like it should be far easier to track then say a White Chevy or Ford pickup. I don't think it would be hard to make a list of every Black Nissan Pro 4X Frontier in that part of Texas and to question every owner, or see if any of them knew Liz or could be connected to her. I think there is a far less chance this truck was a rental for the reasons I posted above of it not being as popular as Chevy/Ford, so rentals would be easier to track. This person may very well have sold, traded, painted, or modified this truck very soon after the murder which would seem like it would stand out. I think if this case & details of that truck were featured heavily on social media throughout that part of TX, it may wind up landing in front of someone with who that truck rings a bell.
Just my 2 cents