Ornata wrote:

What's the best way to get of these critters and bring this tree back
to its former happy state?

In the longer term, try hanging some bird-feeders to attract small
birds to the area. They will eat the aphids.

I am fortunate enough that my ivory-colored garage sits directly in
front of my vegetable garden. I used to have significant aphid
problems. Then one ear the asian ladybugs moved in (they were imported
as home garden pest controls) and I have never had problems since. The
aphids reappear in late october, after te ladybugs have turned in for
the winter. thousands of them overwinter in the garage, since in their
home range they overwinter in ivory-colored rock cliffs.
So, if you can find a way to maintain a ladybug population, they will
eat nothing but aphids.