Nick, do you use the normal definition of a Boltzmann brain?

It's supposed to be a mind which comes into existence by sheer random chance. Additional complexity - such as would be required for some support structure (e.g. an actual brain), or additional thinking without a support structure - comes with an exponential probability penalty. As such, a Boltzmann brain would normally be very short lived.

In principle, though, there could be so much space uninhabitable for regular civilizations that even long-lived Boltzmann brains which coincidentally have experiences similar to minds in civilizations outnumber minds in civilizations.

It's not clear whether you are worrying about whether you already are a Boltzmann brain, or if you think you are not one but think that if a Boltzmann brain took on your personality it would be 'you'. If the former, I can only suggest that nothing you do as a Boltzmann brain is likely to have much effect on what happens to you, or on anything else. If the latter, I think you should upgrade your notion of personal identity. While the notion that personality is the essence of identity is a step above the notion that physical continuity is the essence of identity, by granting the notion that there is an essence of identity at all it reifies the concept in a way it doesn't deserve, a sort of pseudosoul for people who don't think they believe in souls.

Ultimately what you choose to think of as your 'self' is up to you, but personally I find it a bit pointless to be concerned about things that have no causal connection with me whatsoever as if they were me, no matter how closely they may coincidentally happen to resemble me.