a better expression or single word for "developer usabilty"

You may be looking for user–friendliness. The developers in question are users in this context.
Perhaps you can even adapt it as developer-friendliness to make it more explicit.

M-W:

user–friendly adjective

: easy to learn, use, understand, or deal with : user–friendly software

• user–friendliness noun

Wordnik:

developer-friendly

Examples:

But Apple changed the game in 2008 with its developer-friendly App Store, which now hosts some 350,000 apps that have been downloaded over 10 billion times on the iPhone, iPod touch and iPad.

The San Francisco-based startup hosts all the computing functions of its web developer-friendly telephony service on Amazon Web Services, a business model that enables Twilio to differentiate itself even further from its voice API cohorts.