Advanced computation doesn't happen at those temperatures, for the same basic reasons that advanced communication doesn't work for extremely large values of noise in SNR. I was trying to illustrate the connection between energy flow and temperature.

And I was trying to illustrate that there's more to life than considering one cold brain in isolation in the void without asking any questions about what else all that free energy could be used for.

So, now consider moving the matter around. What would be the point of building a dyson sphere? You don't need more energy. You need more metal mass, lower temperatures and smaller size. A dyson sphere doesn't help with any of that.

A Dyson sphere helps with moving matter around, potentially with elemental conversion, and with cooling. If nothing else, if the ambient energy of the star is a big problem, you can use it to redirect the energy elsewhere away from your cold brains.

But doing anything with the star would probably take a very long amount of time, so it's only relevant in non-transcendent models.

Exponential growth. I think Sandberg's calculated you can build a Dyson sphere in a century, apropos of KIC 8462852's oddly gradual dimming. And you hardly need to finish it before you get any benefits.

So it may be worth while investing some energy in collecting small useful stuff (asteroids) into larger, denser computational bodies. It may even be worth while moving stuff farther from the star, but the specifics really depend on a complex set of unknowns.

You say 'may', but that seems really likely. After all, what 'complex set of unknowns' will be so fine-tuned that the answer will, for all civilizations, be 0 rather than some astronomically large number? This is the heart of your argument! You need to show this, not handwave it! You cannot show that this resolves the Fermi paradox unless you make a solid case that cold brains will find harnessing solar systems' energy and matter totally useless! As it stands, this article reads like '1. reversible computing is awesome 2. ??? 3. no expansion, hence, transcension 4. Fermi paradox solved!' No, it's not. Stop handwaving and show that more cold brains are not better, that there are zero uses for all the stellar energy and mass, and there won't be any meaningful colonization or stellar engineering.

There's also KIC 8462852 of course. If we assume that it is a dyson swarm like object, we can estimate a rough model for civs in the galaxy. KIC 8462852 has been dimming for at least a century. It could represent the endphase of a tech civ, approaching it's final transcend state. Say that takes around 1,000 years (vaguely estimating from the 100 years of data we have).

Which is a highly dubious case, of course.

we probably can't yet detect stars that already dimmed and then stabilized long ago.

I don't see why the usual infrared argument doesn't apply to them or KIC 8462852.