Hi Nio82 i did this your step a year ago
I changed original CPU with dual cpu (two Xeon x5680), 96 Gb DDR and also a 6600 (with MAC EFI) and i used it a bit with OSX Monterey
I had also an USB 3 PCIE, a Samsung NVME and a Broadcom PCIE Wifi
Some time ago i changed 6600 gpu with a RX 580 8gb (no MAC Efi in it) and i installed Ventura OS
Both OS work well but i cant advice to invest so much on it...
i am selling it and i can say it wasnt for me a good deal (but very happy to study it and solve problems to have it working
With rx 580 i have also installed Sequoia, i think best OS for it are Ventura (or Monterey)