Karaze If i'm not mistaken the default behavior of grapheneos is to 'spoof' a random mac address each time u connect to a acces point. U can change this behavior to randomize your mac address to per connection (default i think), per acces point, and set it to use your device it's hardware mac address
Changing IMEI is not possible, and even if it was possible, there's not much of a benefit as there is other identifying information sent to a cell tower, and would only make u stand out, and probably signal your provider you are breaking the law presented with your identity on a silver plate. Unless you know exactly what u are doing and make sure that other identifying data has never been connected to u in any way.