Learn English – Understand prefix out-

The Online Etymology Dictionary reports the following:

early 13c., amasian "stupefy, make crazy," from a-, probably used here as an intensive prefix, + -masian, related to maze (q.v.). Sense of "overwhelm with wonder" is from 1580s.

So the a- is a prefix here, but it intensifies instead of negating. Wiktionary lists the several uses of a- and notes the Greek-based meaning you cite as the only one that remains productive today.

Cambro- is the traditional form; the OED defines it in part as:

…used as combining form in the sense ‘pertaining to Cambria, Welsh’.

It’s never been common, but has clung on tenaciously over the years, in cambrophone, cambro-centric, Cambro-American, and the like. (It’s much more common in geological use, where it refers to the Cambrian Period.)

However, few apart from classicists or historians will understand it. An alternative option is Cymru- or Cymro-. This is less traditional, but usage on the internet and in British newspapers suggests it’s probably about as common today; and it has the great advantage that most Brits, and certainly anyone who’s lived in or near Wales, will understand it immediately. So if you don’t mind creating etymological Frankensteins, this is what I’d recommend.