White Knuckle - Not even a option. Used to ride there back in the day. Small track that I can even ride.
Cycle Ranch - Best dirt around for south Texas. Been through many owners over the years. Seems to struggle from time to time. Would required major upgrades to handle traffic on race day.
Zars - Huh? It's all single track trails. I ride there quite often.
ADVMX - I like this track in Austin. Again IMO not really fit for a pro race without major mods to the track.
Not sure about the others My brother used to ride MX and I will say North Texas probably has better tracks. Keep in mind, pros used to run Freestone years ago. AMA pulled the date for various reasons.
Peak months to ride. I guess it depends how tough you are. I would say October - May(ish). After that is gets pretty hot and the humidity kicks in.