Yes, at the entrance, to the left, passing the main gate.
Yes, there are lockers you can rent throughout the park
Yes by entrance to the left
Yes they do. When you walk through the front gate they are on the left going towards "Mexico"
Yes, there are lockers
No but when you get on a ride you can put your things in the cubby
They have some by Mr.Freeze, but I think those are just temporary while you get on the ride. To answer your question though no, they do not have lockers you can use for the whole day.
At the left of the entrance, in specific park areas, and cubby areas per ride depending on the ride as well. May I suggest trunk space, as it is readily available for personal vehicles into the park and being monitored by security (on an individual car's basis)
They do actually have lockers in the front as well as over by Mr. freeze. Stop by Guest services after you get past security and directly before going through the turnstiles♡ https://m.sixflags.com/overtexas/plan-your-visit/park-services
Thanks! Your answer is awaiting moderation.