B450 Pro4 5 beeps no display

I wouldn't mess with downgrading BIOS versions. I also highly doubt it's RAM incompatibility.
The 3600 should support all the same RAM that the older architecture 2200g did.
Manufacturers don't test every single RAM model and brand with each architecture.
Generally, if it's compatible with an older model CPU it will also be compatible
with newer ones.

If you want to go ahead and try more things, before you mess with BIOS downgrades etc
I would suggest trying various BIOS settings with the 2200g installed that have
to do with the GPU and PCIe.
Maybe try and set the PCIe to PCIe 2.0 or 3.0 manually then reinstall the 3600.
Ideally I would have liked to see what happens if you use the other PCIe slot
but you said that isn't an option. I have a suspicion that the system will output
a display if you do.

I am off to bed but I will check back here in the morning.

Good luck

Found part causing issues
after yeeting b450 asrock(am4)
for Z370 gigabyte (lga1151)
PC with CPU not having iGPU posted no issue
so looks like mobo was faulty

yea went back to using i5 9400f and not going to touch amd any time soon