See, if the universe is eternal, and by the universe I mean the most fundamental blocks of matter/energy, then us existing again is guaranteed.
Heat death, like you said, doesn't mean nothing will exist, something will always exist, it's just that we don't know for sure the properties the universe will exhibit by then, it could be the case that some properties only emerge under the conditions of the heat death, ones that can shape matter into how we know it.
I also agree that us, as well as our memories and thoughts, are nothing but a specific arrangement of matter.
The question I struggle with though, is who am I? Can I exist under different circumstances that those I was born into? Could "I" have existed in a different body, circumstances, culture, and still be the same me? When I say the same me, I mean the same conscious entity. I'm not the same as my younger self in terms of so many things, but my current self and younger self are of the same conscious entity, one conscious agent.
The question is, how many different lives, if any, I can exist in and still be me? What if me can only exist within the exact same circumstances I exist under now? This means that if I ever come to be in the far future, I'll be going through the same story again, unless me can exist under other circumstances and still gave the same agency.
It's basically the problem of identity in philosophy.
My current belief is the following: We become conscious at a certain point in time, so we're not conscious right out of the womb. The moment we become conscious is the moment "I" us created, i.e. the agency.
If that is true, then as long as the exact atoms formation we had when we became conscious can exist under different set of events, then there is a possibility of you, for example, existing under better circumstances next time.
However, if the atoms formation we had when we became conscious can only exist under the circumstances that we had in this life, then it's unfortunate news, cus this mean whenever we exist, the same story will be told again, and again.