The effective nuclear charge, Z, for helium, is 1.5 due to the presence of the fellow electron, so the effective "alpha"=Z*q^2/(hbar*c), for the helium Bohr model is 1.5x larger than for hydrogen. According to the theory above, the temperature of helium should be 1.5^4=5x that of the CMB (i.e., of hydrogen); this agrees well with the far CIRB peak.
The effective Z for oxygen, acting on its inner S1 electrons, is, because of "P2 orbital" electrons, only roughly 0.005 less than 7.5. Carbon, nitrogen and neon, also abundant in the universe, broaden that peak. For oxygen, 7.495^4*2.8016=8841 K, hotter than the near CIRB+cosmic optical background peak (which is downshifted by dust and by cosmological redshift), but equal to the surface temperature of typical bright stars. It is the inflection point of the Hertzsprung-Russell diagram (plotted on log-log paper)(see below).
For iron (neglecting P, D, and outer S orbital electrons), T=1.15 million degrees and the peak is 0.30 keV. The soft cosmic X-ray background lacks Planck shape and is said to range mainly from 0.5 to 2 keV or, according to another authority, from one to ten million degrees. Copper and zinc also fall in the lower end of the range.
For lead the peak is 44.1 keV, for thorium 69.9 keV and for uranium 78.2 keV. So, the heaviest stable elements match the (also somewhat irregular) hard CXB peak of 40 keV. For these heavy elements, use the relativistic formula 3*k*T=m*c^2*(gamma-1)*(1-gamma^(-2)), where gamma=sec(arcsin(beta)) and beta=(Z-0.5)*alpha.
