There is a fundamental reason why it can't decay past Fe (to Si). Google binding energy of iron.
Um, uranium decays far past Fe, to He actually. Losing a lot of binding energy compared to emitting Si, or O, or C.
All light isotopes of uranium actually undergo cluster decay. So do U-232 (Ne-24, branch ratio 9e-12), U-233 (also Ne-24, branch ratio 7e-13), U-234 (also Ne-24, branch ratio 1e-13, as well as Mg-28, branch ratio 1,4e-13), U-235 (also Ne-24, branch ratio 8e-12) and U-236 (Mg-28, branch ratio 2e-13).
Whereas for some reason U-238 does not undergo cluster decay. The minor fates are spontaneous fission (branch ratio 5e-5), long range alphas (region e-8), other branches of "ternary fission" (region e-9, most widespread third product being triton) and double beta (branch ratio 2e-10).
Why does U-238 not undergo cluster decay?