When I lived in Virginia there was a guy, a healthy young adult, who was bitten on the hand while cutting out some river cane. The immediate symptoms were as described above. At the hospital he was given several vials of Crowfab plus other stuff they do on bite victims. He had a hospital stay and almost lost his hand. He detailed his fight weeks later and plenty to say about copperheads and their bites. For one thing the snake was in the cane resting at head height. The other was about what he called "his close call" and some permanent damage. A rattler bite would have been exponentially worse.
Thing about venomous snakes (applies to spiders, too) is the critters size, age, health and whether or not it recently used its venom. Snakes, especially, deliver "dry bites" at least half the time. The reason? It takes a great deal of energy and time to replenish their stock; plus they often release only a portion of that venom (it's that critical to their survival). Any venomous bite is, like a gunshot, not always the same in effect. Some victims survive, some don't, some suffer permanent damage and some don't. In other words it just depends. The thing about increasing toxicity of deep SE canebrake rattlers is that the cause is unknown. They are the same species as the timber rattler (crotalus horridus) but colored differently and only found at low and more moist elevations; hence, "canebrake". The species, both varieties, can be black or something in between. So it pays to know your snakes. Luckily, when you see a rattlesnake, you'll know it. I like snakes and have been around them, caught them, studied them, been bitten (non venomous) and protected them. One thing that also seems to occur as they get larger (Eastern diamondbacks, for instance) is that they get more sluggish and tolerant. This is not to say less deadly , just more hesitant to bite. In my experience this does not happen with the pugnacious pygmy rattler; but they are a different genus completely, not "crotalus". But those little guys stay tiny always.