Well, you've certainly tried changing a lot of components before coming here for help! Two boards at this point.
1) If you still have access to the other system, update the 4090 VBIOS to the latest version if one is available: https://nvidia.custhelp.com/app/answers/detail/a_id/5411/?=&linkId=100000163736896
2) Use USB Flashback to update the Z790 Apex to the latest UEFI revision: https://rog.asus.com/uk/motherboards/rog-maximus/rog-maximus-z790-apex-model/helpdesk_bios/
3) Test the system outside of the chassis
4) Potentially look at trying to RMA the CPU to the etailer/Intel or get a system Integrator to test it for you.