The series of unfortunate events went something like this:

- pull out CPU from socket in the wrong way (red herring)
- in the process of removing/reinstalling the CPU, the PCIE riser cable goes from a marginal connection to one that no longer works
- machine no longer boots
- attempt to re-seat cable, but attempt wasn't good enough as it's really difficult to manipulate when the machine is fully installed in the case
- become increasingly convinced that the problem is the CPU even though the lights on the motherboard get stuck at the VGA step (initializing the VGA is handled directly by the CPU over PCIE so a problem with the CPU could manifest itself as being unable to initialize graphics)
- reset the BIOS, which changes the default PCIE protocol to PCIE 4.0, which the riser cable does not support (apparently there's no way to fall back to lesser protocols if both devices negotiate PCIE4 at initialization?)
- swap GPUs for a GPU that I thought was good, but appears to be bad or at least incompatible with ryzen 5000 systems
- install CPU in old motherboard with different RAM, with said bad GPU, machine gets stuck at DIMM training for an improbably long time
- become convinced problem is the cpu
- order new CPU and install it into current motherboard, problem still persists
- plug full-fat GPU into old motherboard with old CPU, machine boots just fine
- reconsider every choice in my life until this point
- plug full-fat GPU into current motherboard with new CPU and reconfigure the PCIE link to 3.0
- rebuild entire system, boots fine