I want to make a persistent implant/malware that survives OS reinstalls.
Look up Absolute Computrace Persistence. It's there by default in a lot of BIOS images, but won't survive a BIOS reflash with an image that has the module stripped out (unless you have the "security" of Boot Guard, which will effectively make this malware mandatory!)
I’m more interested in demonstrating how important hardware root of trust is.
You mean more interested in toeing the line of corporate authoritarianism.