The last

...it was beautiful. And that is good.

~fin

Not entirely sure if I interpreted your intentions right when I tried to write an answer. In particular, I'm confused by the line "I could create just a little more hedonium". My understanding is that hedonium refers to the arrangement of matter that produces utility most efficiently. Is the narrator deciding whether to convert themselves into hedonium?

I ended up interpreting things as if "hedonium" was meant to mean "utility", and the narrator is deciding what their last thought should be - how to produce just a little more utility with their last few computations before the universe winds down. Hopefully I interpreted correctly - or if I was incorrect, I hope this feedback is helpful  :)

I really hope that humanity and its descendants think about what they want to experience last a very long time before it actually happens. I personally would like the last experience in the universe to be something akin to being wrapped in a warm blanket and very slowly falling asleep, an experience of comfort (and not much discursive thought), but I really hope we'll talk about this beforehand!

Minor nitpick:

And so we have ever less energy to simulate our brains. It gets ever harder. With less energy, we are forced to slowly shut systems down.

I think that the problem is not lack of energy, but lack of energy gradients or negentropy to get any useful work out of.

H/t to Qualia Computing: 

"Desiring that the universe be turned into Hedonium is the straightforward implication of realizing that everything wants to become music.

The problem is… the world-simulations instantiated by our brains are really good at hiding from us the what-it-is-likeness of peak experiences. Like Buddhist enlightenment, language can only serve as a pointer to the real deal. So how do we use it to point to Hedonium? Here is a list of experiences, concepts and dynamics that (personally) give me at least a sort of intuition pump for what Hedonium might be like. Just remember that it is way beyond any of this:

Positive-sum games, rainbow light, a lover’s everlasting promise of loyalty, hyperbolic harmonics, non-epiphenomenal bliss, life as a game, fractals, children’s laughter, dreamless sleep, the enlightenment of emptiness, loving-kindness directed towards all sentient beings of past, present, and future, temperate wind caressing branches and leaves of trees in a rainforest, perfectly round spheres, visions of a giant ying-yang representing the cosmic balance of energies, Ricci flow, transpersonal experiences, hugging a friend on MDMA, believing in a loving God, paraconsistent logic-transcending Nirvana, the silent conspiracy of essences, eating a meal with every flavor and aroma found in the quantum state-space of qualia, Enya (Caribbean Blue, Orinoco Flow), seeing all the grains of sand in the world at once, funny jokes made of jokes made of jokes made of jokes…, LSD on the beach, becoming lighter-than-air and flying like a balloon, topological non-orientable chocolate-filled cookies, invisible vibrations of love, the source of all existence infinitely reflecting itself in the mirror of self-awareness, super-symmetric experiences, Whitney bottles, Jhana bliss, existential wonder, fully grasping a texture, proving Fermat’s Last theorem, knowing why there is something rather than nothing, having a benevolent social super-intelligence as a friend, a birthday party with all your dead friends, knowing that your family wants the best for you, a vegan Christmas eve, petting your loving dog, the magic you believed in as a kid, being thanked for saving the life of a stranger, Effective Altruism, crying over the beauty and innocence of pandas, letting your parents know that you love them, learning about plant biology, tracing Fibonacci spirals, comprehending cross-validation (the statistical technique that makes statistics worth learning), reading The Hedonistic Imperative by David Pearce, finding someone who can truly understand you, realizing you can give up your addictions, being set free from prison, Time Crystals, figuring out Open Individualism, G/P-spot orgasm, the qualia of existential purpose and meaning, inventing a graph clustering algorithm, rapture, obtaining a new sense, learning to program in Python, empty space without limit extending in all directions, self-aware nothingness, living in the present moment, non-geometric paradoxical universes, impossible colors, the mantra of Avalokiteshvara, clarity of mind, being satisfied with merely being, experiencing vibrating space groups in one’s visual field, toroidal harmonics, Gabriel’s Oboe by Ennio Morricone, having a traditional dinner prepared by your loving grandmother, thinking about existence at its very core: being as apart from essence and presence, interpreting pop songs by replacing the “you” with an Open Individualist eternal self, finding the perfect middle point between female and male energies in a cosmic orgasm of selfless love, and so on."

May all be well.