Does the great wheel model preclude moral agency?
Well...
Some versions of it (especially both Dragonlance and the Realms) claim that Balance Between Good and Evil is good. And that does wreck moral agency.
...this is a good start on why it does, but there's more to it than that. Specifically,
alignment determinism and, at least for FR, the Wall of the Faithless.
In the Great Wheel, people
know that you go to the afterlife associated with your alignment. Not only is there no doubt about this, but the perks and benefits of each possible "destination" are well-established facts independent of your
intentions with your behavior. In other words, every person who dies, goes to the place that
actually conforms to the values they championed in life (even if they did not openly
espouse said values to others, or failed to realize that they were championing said values). Because this is known, people are encouraged not to make moral choices
because of morality, but rather, because of the
perks you get for dying with that alignment. Step 1 on the journey to infinite power (or comfort, or destruction, or freedom, or whatever else) is making sure you live a mortal life that will send you to the correct afterlife for your goals. That causes a removal of
moral agency (not agency in general, just the
moral aspect of it) because it removes the
moral component of judgments: you aren't thinking
ethically at all, but rather instrumentally. You are instead encouraged to think morally only in the sense that you want to avoid
intending to follow <X Plane>'s moral goals but accidentally end up following <Y Plane>'s goals instead.
The Wall of the Faithless actually makes this
worse, because it actively turns morality into a
protection racket. The gods cease to be paragons of values (whether good or evil or anything else), and instead become mafia dons coercing worship out of mortals, with the threat of excruciating pain and gradual soul destruction. Under these lights, it becomes even more totally instrumental thinking: pick the god you will find it easiest to avoid "betraying" (since the False are punished too, just
usually in a less horrific way than the Faithless) with an afterlife you can accept, follow them with the minimum effort to fulfill your end of the protection racket, then go to the afterlife you selected.
In a cosmos with no inherent moral alignment woven into its literal structure, and especially one where the afterlife is left in doubt (as it is in the World Axis), the choice to engage in certain moral behavior is
necessarily more than just a calculation of expected utility. You are actually choosing morally; you
can't be just making a calculation.
And count me in for preferring 4e slightly to Eberron and both those to just about any other D&D cosmology by a large distance. An interesting thing abut the 5e cosmology is for all it calls itself the Great Wheel it's the 4e World Axis with the Astral Sea broken out into the outer rim of the old Great Wheel.
Yeah, the "new" Great Wheel is frankly at least half World Axis, just
very insistent about how it's Definitely Still Traditional, We Promise.
A big thing no one else seems to have mentioned is the elemental planes.
Realizing that the problem with elemental planes was that they were elemental planes was a eureka moment for me and the Elemental Chaos was an excellent jumping off point for my designs with keeping the 'elements' playable and survivable in mind.
Frankly, part of why I
intentionally didn't mention them is some people think this is The Worst Thing Ever, that having the "pure" elemental planes is super duper important. So I just didn't want to kick that particular hornet's nest.