What the problem you are having with Monterey? You other post is not clear, but for some cards that do not follow the exact Apple personality, not all outputs work with Monterey.
For the pre-boot configuration not working, unless you did something really wrong injecting EnableGop to the Mac Pro BootROM, your most probable issue is the GPU firmware being modified/invalid/etc. Verify if your GPU have the correct GPU firmware, you can get almost all GPU firmwares from TechPowerUp, see the post below for pointers on what to do:
I very recently solved the same situation you seem to have: 5,1 bootROM with EnableGOP, OCLP, and a (used eBay) Sapphire Pulse RX 580 8GB with dual BIOS switch but no boot picker unless AMD Inject GOP was selected in the advanced OCLP config. There is a method for flashing GPU directly on a...
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