Failed installation, boot blocked.

I've tried installing Monterey on a 2009 Mac Pro with firmware 5.1 from High Sierra. Upon rebooting to start the installation, I get a prohibited icon when trying to boot from the USB installer. If I attempt to boot the previously installed High Sierra system, I see three options at startup: BootKicker.efi, Openshell.efi, and Reset NVRam, none of which allow me to proceed. I've tried creating a new High Sierra USB installer, but when attempting to boot from it, it hangs. The same happens when trying to enter the recovery system; the Apple logo appears with the loading bar, but it doesn't progress—I've left it for hours with no change. In fact, I tried booting from another mechanical hard drive with High Sierra installed from another identical Mac Pro, and not only does the same thing happen, but the disk also makes the classic sound of shutting down or stopping spinning, freezing up. I've removed the BIOS battery for half an hour and upon the subsequent boot, reset the NVRam using the Option+Command+P+R commands multiple times, but there's no change. I've completely formatted a disk on another Mac Pro and tried inserting it to bypass this block, but everything remains the same. In summary, neither the original system nor any USB with new installers allows me to boot the Mac. Any help?