The phrase axis mundi is used frequently in archaeology and art history to describe certain places as a "world center" or "center of the universe" in Indigenous or ancient/historical thought. How would this phrase be correctly pluralized? For instance, "By the early first millennium BCE, new urban centers developed across the region and were conceptualized as axes(?) mundi(?)." Any insight would be useful!
Yes, the nominative plural of axis is axes.
Mundi 'of the world' is the genitive singular of mundus 'world', and you probably wouldn't pluralize it in most contexts (presumably there's still only one world), but should it become necessary to do so, the genitive plural is mundorum 'of the worlds'.
Axes mundi reads correctly to me, for your purposes.