To me signing up for superpower surgery can raise "if there exists a me, it is superpowered" to arbitarily high but it would at the same time lower "after the surgery there is a me" at the same rate.

This would kinda leave a funny edgecase where a brain in a vat could correctly conclude that "I don't exist" if it finds evidence that nothing that fits it's self image exists in the world (ie beings with hands etc). It would still be blatantly obvious that something is having the thought and it would be really nice if "I" would refer to that thing regardless of how you picture yourself.

You could have a situation where you are a brain in a vat in your lap with all your sensory inputs being conveyed by a traditional body. It would still be pretty challenging to determine whether you are your skull or the fishbowl in your hands. Maybe the multilayered use of me in the previous sentence points at the right way? So what happens to the thing you are now (or your extended-you) is a different question on what you will become (your core-you). That way the only way for core-you to terminate would be to not to have thoughts. Breaking the extended-you would thus not terminate your toughts and core-you would still be core-you.