Sorry if this sounds stupid, but just to be clear: "Perform CMOS Reset" = momentarily shorting across the 2 jumper pins above the pin block for power, HDD light, etc?
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I actually tried this a couple times. After 5 beeps, I'd shut the system down (power switch on PSU), then take a small screwdriver & momentarily jump across the 2 pins, then try to power on the system again. Wasn't able to get past the 5 beeps so I could get in to apply all prescribed BIOS settings.
I may have a ghost in my machine LOL I went back to F6 - had to remove GPU, then connect monitor to iGPU and boot into Windows to use the Gigabyte @BIOS app to update F5 > F6.
As I recall, I had F5 OoB. I assembled all the components into the case, including m.2 drive and my RX580. Had the same issue with the 5 beeps. So my go-to troubleshoot is to always simplify to the bare bones. Pulled the GPU, pulled the m.2 drive, used a spare SATA 840 EVO SSD and did a quick install of Windows 10 Pro. It was then I discovered the Gigabyte @BIOS app & the F6 version. After the update, I was able to get that initial build of Catalina up & running.
I've got time (I'm on vacay this week), so I'm happy to try F5 again. Another quirk: with F6, CSM "Disabled" would "stick" during the first few reboots after I went back to F6 & Windows + iGPU. BUT...the minute I put in my GPU, and returned to the BIOS, CSM was back to "Enabled." Strange things going on....