I keep telling people not to pay the DDR5 tax as it's not that big of an improvement. The need to go DDR5 right now is AMD exclusive at this point and might become an issue with MTL/ARL 23-24 release cycle depending on if Intel leaves the chipset open to DDR4 or not.
The only benefit to DDR5 right now is to spend more money on the RAM / Board.
As to the BIOS/UEFI flashing there's ways around needing a CPU installed with some boards. I haven't personally tried it with ADL / Z690 but, my laptop released a new UEFI and EC flash and the EC needed to be done from USB and used EFI file structure to do the flash itself and some CLI / DOS commands to execute it. I didn't find an OEM program to do this but used another one from another reseller that worked just fine to boot from and execute the command.
There's inherently always a gotcha moment when transitioning from a board made for X when using Y CPU. If the case needs the generation CPU it was designed for I've heard of rental options from some where you put a deposit down on the CPU and when you're done you send it back. I would think most boards though can flash from USB w/o a CPU installed though.
You can update the BIOS without a CPU to make your Motherboard ready for newer Processors. Here's how to use the so-called BIOS Flashback Feature.
Introduction If you have a modern DIY desktop PC that uses a motherboard from one of the large motherboard vendors from Taiwan (such as ASUS, ASRock, BIOSTAR, Gigabyte, or MSI), there is a good cha…