Before you start I would first make sure you have the latest macOS and updates applied then backup your stuff onto an external drive using TimeMachine. Once done go into FindMy and remove your system from your group. And disable any protection you have on the drive.
Last step here is reformat your systems drive and install a clean copy of macOS. I would recommend you boot up under an external bootable drive (setup just for this task) to achieve this so there is no fingerprints of yours on the system.
Now the fun stuff! Remove the logic board and be very careful with the power button/TouchID ribbon cable as its very easy to damage. Use care as its easy to snag the other cables when you put in the replacement logic board. Using the external bootable drive boot up and format and install a clean copy of macOS here we want to be sure nothing is on the drive (i.e. spyware or other persons stuff). That should do it. Now add your system into your FindMy group to make sure there is nothing hanging on it.