>> I guess my main concern versus C is portability and ease of integrating libraries or exporting a usable C API, and secondarily the quality of the optimizer.

Interfacing with C APIs / libraries is really easy and portable across Ada implementations:

https://learn.adacore.com/courses/intro-to-ada/chapters/inte...

http://www.ada-auth.org/standards/22rm/html/RM-B-3.html

The quality of the optimizer depends on the Ada implementation.

GNAT, the free software Ada implementation, uses the GCC backend so it is pretty good:

https://www.getadanow.com/

https://www.adacore.com/gnatpro