Interesting speculations. So much of the sense of a soul or a spirit depends on an idea that an individual self exists. Am not at all sure that is the case.

Like your example of the brain disorganizing itself, not unlike Marcus Aurelius' thought "Soon everything and everyone I know will be forgotten." Am rather a fan of this trend in thought. Can see absolutely no use for continuing to know that one plus one is two, that the sky is called blue, in English, or that Donald Trump was elected President in 2016. All that is basically junk.

If there is no self, there is still the fact that we are or were a part of the universe, interrelated with it, nature and the universe do not exist outside of ourselves but as a part of us. A unified field theory of life, energy, and matter.

The intellectual tricks we learn here are useless outside of our lives here. They too get dissolved and forgotten.

Yet, in a universe where everything is either recycled or repurposed, it is likely that something basic to us survives, whether it is our fears, our curiosity, or some innate quality that defines us, continues on its journey through the stars to who knows what and who knows when.

Just some thoughts on the subject from a different point of view.