Thanks pastrychef for the EFIs and detailed instructions!!
Since I've found FCPX to export about 15% faster on iMac19,1 than on iMacPro1,1 (with no overclocking - Activity Monitor shows active cpu on iMac19,1 and idle cpu on iMacPro1,1) I prefer to use iMac19,1 but I am having the following problem:
With IGPU enabled in Bios and using iMac19,1 system definition, after booting windows 10 and then going back to Catalina, I get the same forbidden symbol at beginning of macos boot just like when I switch system definitions from iMacPro1,1 to iMac19,1 (and vice versa) and then need to clear CMOS to proceed. The thing is, this doesn’t happen on iMacPro1,1. Using F12 and booting a Windows Boot Manager booting option in Bios will trigger the forbidden symbol every time after returning to Catalina... I tried using clover to boot windows instead of F12 boot selection in Bios and it worked initially (to get back to Catalina with no forbidden symbol) but forbidden symbol returns after a few windows <-> macos switches. Once I get back into Catalina (clear CMOS again) I can then reboot, shutdown, even edit Bios, and still boot Catalina fine, just once I boot into Windows I need to clear CMOS again to boot Catalina.
I know you say in post#1 that enabling IGPU in Bios “may cause problems booting at times” but I thought it was worth mentioning this in case you have a suggestion, any ideas?
I’m on Bios F9k, and my EFIs are your clover r5120 except for I've added agdpmod=pikera (for 5700 XT), and on my copy for iMac19,1 EFI I've removed AppleALC (to restore wake from sleep functionality) which is ok for my build since I use a USB audio interface. When I swap iMacPro1,1 <-> iMac19,1 I only change the Bios setting IGPU Auto <-> Enabled (After clearing CMOS and restoring Bios settings profile).