Why do pair production and photodisintegration have such different effects in collapsing supermassive stars?
A bit of wild speculation that I'll need to check.

Photodisintegration produces simpler particles. These simpler particles are able to "neutronise" (i.e. undergo inverse beta decay/electron-capture) with electrons, because the energy threshold for neutronisation is much lower for free protons and alpha particles than it is for say oxygen or iron nuclei. This process removes free electrons from a core that is supported by electron degeneracy pressure and it collapses.

Pair production converts photon energy into electrons and positrons. The net effect will be to take kinetic energy out of the gas and turn it into rest mass. This will lower the pressure, but an increase in density/temperature can restore it.

I think the process of neutronisation/electron-capture is the key difference. The threshold electron energy for neutronising a free proton is 1.3 MeV, but is about 11 MeV for a proton inside an oxygen nucleus, so photodisintegration is required to trigger collapse (or the Fermi energy of the electrons reaching the neutronisation threshold).