Doesn’t each processor chip (the CPU hardware) have a unique serial number. Couldn’t that be embedded in various outgoing messages?
I’d say don’t use a computer or phone if you are really concerned about being snooped on. Maybe not mail either. Try to communicate with a wink of the eye.
MAC ids on network and motherboard interfaces are often in EEPROM and capable of override for spoofing purposes. Some vendors use a MAC ID as a license key. That was viable when the manufacturer generated an immutable key for the network interface. That's not the case anymore.
Finding ways to track your activity will be a perpetual cloak and dagger effort. New ones will crop up soon after you've nailed the ones you discovered.