There is a long history of connectionist attempts trying to ballpark the brain compute to constrain AI timelines, going back to von Neumann/Turing/Good. The most recent one would be https://www.openphilanthropy.org/brain-computation-report You can see in Figure 1 that your 10^12 steady state is the very low end. If you're interested in seeing where your envelope estimate differs from the others, well, it has the references.