MP 1,1-5,1 Early 2009 MP w/Mojave goes to black screen when trying to get to boot options

[phdillard]

I'm trying to get my recently-updated-to-Mojave 2009 MP to go to the boot options screen so I can use the usb boot I made for Catalina per dosdude1's instructions, but holding Alt just results in a black screen that never changes and I have to power cycle the computer to get it to load to the normal desktop. Here's what I've recently done:

updated firmware from 4,1 to 5,1
updated from El Capitan to Sierra then to High Sierra and I did install the HS firmware
then I upgraded the stock 2.66 quadcore with a x5675 hexacore
upgraded ram from 16GB 1066 to 32GB 1333
swapped out the stock radeon 4870 with a k600
then updated from HS to Mojave, no patcher, just a straight update

All 3 OS updates were in-place, no patching. My primary boot drive is a 480GB SSD that shows as apfs in disk utility. About This Mac recognizes all the new hardware correctly and everything seems to be working great. I just can't get to the screen I need to proceed with the Catalina install. I'm assuming an issue with the graphics card, or maybe I should have installed patched Mojave? Any ideas are greatly appreciated at this point.

 

[tsialex]

swapped out the stock radeon 4870 with a k600

You GPU is macEFI flashed?

 

[phdillard]

You GPU is macEFI flashed?

As far as I know, no it isnt. But I had read that the k600 is compatible through Catalina and possibly Big Sur, so I assumed it'd be fine.

 

[tsialex]

If the GPU is not macEFI flashed, no pre-boot configuration support, so, no boot screens. That's your problem.

You can try OpenCore if your GPU correctly supports GOP, but most GPUs of this era (Kepler family of NVIDIA GPUs) are pre-GOP.