A related question - were I to plant a new fruit tree in its place,
would I be wise to stay away from apple (or pear?), so as to not give
the aphids a new similar food source?

IMHO yes, so as also to give yourself an alternative fruit!

Good idea.

New question - what fruit?

I live in the "Inner Richmond" - Just north of Golden Gate Park, due
south of the Golden Gate Bridge - 10th avenue and Balboa, for those of
you who know the city.

Currently, I have:

Mission Fig
3 Apple Trees
(European) Pear
Loquat
Meyer Lemon
Haas Avocado
Summernavel Orange
Calamansi Lime
Keiffer Lime
Pineapple Guava
Passion Fruit Vine

I also have an assortment of herbs and some vegetables (carrots, this
season).

This isn't a large back yard - basically a rectangular lawn with fruit
trees surrounding. I just happen to like planting edible plants (which
is why I'm reading RGE now)

What would make a nice addition? Based on the location, it would need
to be a tree (not the sunniest part of the yard, so to get year-round
sun it will need to be 3 feet tall).