From Oz

[dukealien]

The lone and level sands that fill the bowl  I get an image of a small, decorative bowl here, then the desert.  Better word than "bowl?"
of human understanding do not shift.  but they do, that's why footprints don't last (your second quatrain)
Our pride has no horizons, and our goal  warning - politician's and doctor's (king's?) first person plural here, though it turns out to be benign
is lasting glory; to this world we gift  see below for overall comments on the first quatrain.
such splendid dreams – but who is to recall 
where once we trod, for footsteps always fade
and parchment rots to dust while pillars fall.  "as" instead of "while" for more sandy hiss?
Mortality becomes a beggar’s trade,  I don't get a clear idea from "beggar's trade" - we're poor, arrive with nothing, leave ditto?
yet were our lives not hemmed by earthly thread  the full expression (cliche?) would be "hemmed in" - to me, "hemmed" with "thread" is a finished skirt.  to me, this lacks drama - more "a good, clean death" than "rage against..."  Unfortunate for this otherwise good volta.
would inspiration take that vital turn? 
Or would our patience stagnate us instead?  "stagnate" is not transitive - perhaps "stultify" for "stagnate us?"
‘Tis death that makes the fires of greatness burn.  less archaic to begin with the Reaper - "Death makes the fires of greatness leap and burn," for example, but better verb than "make" (pardon the rewrite)
Take ev’ry breath with eyes cast to the sky – I want a tenser verb than "take" here - "seize," perhaps?  and (again, forgive the near-rewrite) eyes locked on the sky, but better, to include the high angle
there’s time enough for grounding when you die.  I get an echo of electrical grounding as well as burial here - Br. Eng. "earthing," which might also work.  Or not.

Title contains a nice double (or with the Frank Baum universe, triple) reference which extends to the first line.  However, aside from the author's presumed location nothing follows from either the land under Capricorn or in the clouds.  Not actually a criticism, more a missed opportunity -  only Shelley's meditation on vanity and hubris as the starting point here.  Though I may have missed other references?

First off, my notes look like I'm really panning this, but I'm not.  The central thought has been done, but is certainly worth revisiting.  Just a lot of thoughts, hope one is useful.

On the first quatrain:  you work toward two end-rhymes ("goal" and "sift") but their counterparts seem weak.  "Bowl" does not work for me, and "gift" is not a verb except archaically... think "will," "leave," "bequeath," even "give."

Finally (and philosophically, so this para may be disregarded) as Dr. Johnson asserted, the end (goal) of all human endeavor is to be happy at home.  Seeking fame, marble footprints in the sand that cannot be expunged, leads to trunkless legs and shattered visages (that is, busted faces).  Pride and ambition are necessary and human, but can be (and, too often, are) carried to extremes.

A good read.  You can do (and make it) better!