I've noticed a huge flavor profile change as beans age. Things really start going downhill fast at day 5. Is it the way I'm storing the beans (in the hopper, at 60-70% humidity), the roast itself, or is there something I can do to compensate as the beans age?

I've learned to adjust grind to keep consistent pulls of around 25 seconds until blonding at the 1.5oz volume, but I'm wondering if there are also changes I should be making to temperature profile as I go along. The flavor is just so rich and complex 2, 3, & 4 days after roasting, and so bland quickly after that. Any suggestions?

Nope.
What you're seeing is the reality of coffee.
There are some coffees that hit a sweet spot further from their roast date, but they are less common. The vast majority of coffees I've experienced are at their peak (for espresso) between day 2 and day 5.

Every coffee has a sweet spot and soon after it you begin to see dramatic declines.

You can slightly extend the life of the beans by keeping the bulk in a valve bag or mason jar rather than the hopper, ideally under vacuum. However, this is not anything that can do miracles. A blend where the complexity is in flowers and light fruits is going to be dead as doornail in a week no matter what you do. Deeper spice, chocolate, dark berry, current etc flavors may only get started four or five days out of the roaster.

If you look at the SCAA Flavor Wheel, http://www.sweetmarias.com/tastewheel2.jpg on the aroma side, the flavors are arranged by molecular weight. The lightest compounds at the top disappear most rapidly, in a matter of days. As you move clockwise and down, the flavors last longer, with the "dry distillates" lasting almost indefinitely. These flavors also correspond to the degree of roast, since darker roasting drives off the lightest compounds first.

thank you for your responses -- it sounds like i should be buying in half-pound increments twice weekly rather than full pounds once a week

Exactly, or buy from two roast dates if possible (at some roaster/retailers they will have a "cafe" source that is currently at peak and a "customer" source that is at most two days out of the roaster).