Struvite Crystals: How does it form?
As Ed V points out, the conditions of formation of struvite have many variables and the struvite crystals produced are variable. On the one hand, this could be a pleasant exercise of playing with atomic models, but on the other, it may be unproductive unless you were to use computer graphing to model various configurations.
Perhaps you could start with a simpler, but similar compound and develop a crystallization mechanism from that. Ammonium phosphate seems simpler, but isn't, because the third proton of H$_3$PO$_4$ is so weak that pH is still a significant variable, and you are also dealing with diammonium phosphate and double salts (Ref 1).
Another possibility is plain magnesium phosphate, but it appears in several forms and with several degrees of hydration, so it is not a simple crystal formation. (Ref 2) The anhydrous form is too unrelated to struvite. (Ref 3)
Stretching your imagination further, replacing the ammonium with potassium gives an interesting material that has applications in cementing. One article gives a simple picture of the &quot;components&quot; of KMgPO$_4$.6H$_2$O:

The octahedron of Mg(H$_2$O)$_6$$^+$$^2$, the tetrahedron of PO$_4$$^3$$^-$, and the sphere of K$^+$ are objects that could be played with to make a model crystal. The sphere could also suggest how the NH$_4$$^+$ ion (just assume it is almost spherical) fits into struvite. (Ref 4)
Struvite was the basis for a rapid repair patch for roadways some years ago (Ref 5), and has current applications in immobilizing radioactive waste. (Ref 6)
Ref 1. https://en.wikipedia.org/wiki/Ammonium_phosphate
Ref 2. https://en.wikipedia.org/wiki/Magnesium_phosphate
Ref 3. https://byjus.com/chemistry/magnesium-phosphate/
Ref 4. https://pubs.acs.org/doi/10.1021/acs.jpcc.7b01396#
Ref 5. US Patent 3,960,580 J.M. Gaidis et al.
Ref 6. https://www.sciencedirect.com/topics/engineering/magnesium-potassium-phosphate