The on-topic issue is, why I can't install, when the Boot ROM is up to date.
Sorry, but this is not on-topic since you already have the current BootROM release.
Once the BootROM upgrade is already installed to the Mac Pro, Software Updates/macOS install/etc won't touch it again until Apple release a newer BootROM upgrade (which probably won't happen again or Apple would be testing the newer firmware by now with Monterey 12.7.6 betas).
Your issue is OCLP or even something related to the Aura SSD.
Still have to put an original Apple SSD into it to do anything. Thought that would be fixed by now?
That's not because you installed OCLP to the Aura ESP?
Mojave is not compatible with OCLP, OCLP spoofs 2019 Mac Pro to run Ventura and Sonoma, so, since 2019 Mac Pro is not compatible with Mojave, you will not get it to install or work.
Remove OCLP ESP, reset the NVRAM and you can install Mojave. If you need OCLP help, please go to one of the Unsupported Mac threads.