By the same logic, you could ask "what part of a computer 'is' the computer? The CPU? The hard drive? The RAM? The TPM? The power supply? The sum of all peripherals? Etc"

You could ask all kinds of philosophical questions about this, but at the end of the day, there are parts that are easily replaceable and parts that are harder if you want to preserve the identity of a particular machine.

E.g. while RAM, CPU, GPU, power supply etc are all essential for running a PC, you can also swap them out without many problems. In contrast, the data on the hard drives or the TPM might be hard or impossible to restore.

In the same way, I'd still see the brain as the center of the self, because so much cognitive information is stored there.