For mind games: The Quantum Thief by Hannu Rajaniemi.
Plus for a related rabbit hole: the idea that a random pattern can just pop into existence and have consciousness is basically the https://en.wikipedia.org/wiki/Boltzmann_brain concept.
I can plausibly see life extension technology arising that gives me hundreds, or even thousands of extra years of life. It doesn't seem entirely impossible to extend that premise to millions of years, but at billions it doesn't really seem fathomable, and trillions seems to be well beyond what we know about the likely fate of the universe. And once I accepted that inevitability, it suddenly seemed a lot more plausible that my time left was probably closer to decades than even centuries, let alone anything beyond that. There were definitely a fair few existential crises in the weeks and months after finishing that book.
The emergence of a Boltmann brain is on the scale of "all iron stars collapse via quantum tunnelling into black holes"
The next milestone is all matter decaying, and the universe being empty..