I travel this road extensively, and it is full of dips and bumps. Nothing awful, but it does upset the car much.

The car pretty much bobs up and down, jounces and rebounds at different rates. Kinda like your traditional old floaty buick feels like. Thing is, this car is not an old floaty boaty. Its on a revised platform that feels stiffer, modern, and better, you know the feeling. Another way to put it, the car is supposed to feel more snug, composed and what a top of the line sedan should feels like.

I imagine the springs are overdue for a replacement? They probably have supported that car right when it came out of the factory 16 years and 279000 kms ago. The struts were replaced nearly a year ago, and they were sensatracs, so i know they are good. However, I think the struts are working too hard against weak springs, and overall, it upsets the car. The car still rides well over small bumps, but the big and frequent ones upset the car a little. And I mean, it makes sense, the weight of the vehicle must be supported by strong springs, not the struts.

Any opinions here? I might go to a different spring, like a park ave spring or something like that.

Check the ride height of the vehicle. Worn springs sag.

Sounds more like worn out struts/shocks than springs. As Damemorder pointed out, the springs control ride hieght only (excluding aftermarket stuff) and the struts/shocks control ride quality.

edit: Sensatrack even suggests that the shocks/struts be checked every few thousand miles, something like 15-25 thousand? Not a very long life piece of equipment I guess?

Definitely, mine did this and then replaced my struts and I can run her on dirt roads and she responds well and does not float or bounce excessively.

Did you replace the struts with luxury ride or sport ride sensatracs? Because sensatracs, even if the springs are worn, are supposed to continue to respond to shock even when the vehicle is loaded at capacity which would BASICALLY be the same effect of sagging springs.