Well went ahead and did it, has been a bumpy adventure, not for the faint hearted, with it been a new gen.
A note on the bios for the Asrock B450 Pro 4. I was using 1.50 for my 2600X, I upgraded to 1.80 as instructed on Asrock's site (this the best version for zen+, could see much more options to play with). Then to the latest 3.xx (best version for Zen2), then to 4.80 (earliest version that supports 5xxxG chips). On 4.80 I observed nearly all advanced options on my 2600X vanished, the bios was gutted out, the IOMMU grouping was trashed as ACS was removed, I then installed 5.00 the latest public version and Asrock had added back ACS.
So the first issue was when I removed the cooler, the bracket fell back, as the AMD cooler doesnt secure in place without the heatsink attached. I took the back of the case off (old case) and there is no cut out, so had to drag the board out, put something underneath to hold the bracket up and then was able to reinstall cooler on the 5600G. After that reassembled everything.
After powering up, it was a very long post but successful, I checked the bios and observed a fair chunk of advanced options were back, but still many missing, probably half intact, most of the important ones, however no ACS. I think the G chips dont support ACS, its not official anyway but I found someone on reddit who had contacted Asrock who then told him the G chips dont support ACS.
AMD need to publicly disclose this.
I contacted Asrock and pretended to have the USB bug, within a day they sent me a new bios with new AGESA version, it was considerably newer, the public 5.00 has 1.2.0.2, this one is 1.2.0.6 and bios version 5.22.
Still no ACS option however inside was two interesting options called PCIE Alternative Routing. the second one was enumeration. With only the first enabled there was no affect, but after enabling both I now have slightly better IOMMU groups then I had on my 2600X, both cpu routed PCIE slots isolated again. The bios is buggy my second hypervisor boot device isnt visible, on 5.00 it was visible but not selectable as a boot device so buggy on that as well, however luckily all storage devices appear in any booted OS.
The next problem was my NAS VM was crashing on bootup, it worked fine using a generic emulated CPU, after diagnosing each individual new CPU instruction on Zen3, I determined that the PKU instruction is the culprit, seems on BSD guests its not compatible. So I am now emulating an Epyc CPU on BSD guests, and added the missing Zen3 instructions excluding PKU.
I also booted bare metal windows did a cinebench, chip is about 36% higher score vs my 2600X.
The last problem is my monitor a 2209WA has no compatible display type with the onboard outputs, so temporarily I had to keep using the GT 1030, but have ordered a HDMI to DVI-D cable which arrived 5 minutes ago, I will put that in tomorrow.
Hope this helps anyone else who had similar ideas to me for upgrade and use case.
Info on PKU here.
https://en.wikichip.org/wiki/amd/microarchitectures/zen_3