Lookin' good.
Since "Its poison is extremely toxic, and a large amount is injected with each bite", perhaps boost the Con damage a bit more to help differentiate it? Maybe 2d8?
I already doubled the Con damage from a regular Huge Viper's 1d6 to a Wyvern's 2d6.
Do we really need more? 4d6 total is enough to kill a Large dinosaur with a lucky roll. If anything, I'd rather add a racial bonus to the DC since dinosaurs and other Prehistoric Beasts tend to have good Fort saves and the Fanged Python's Con 13 doesn't give it a very good poison DC.
That would also allow us to swap the AF (poison) for something else.
Heck, go for a +4 bonus. DC 19 isn't completely out of the question for CR 5.
Combat Reflexes sounds good.
It's at least as dangerous and tough as a wyvern. CR 6 for sure. Don't think it has enough to be CR 7 quite, though.
Added to Homebrews.