What long term good futures are possible. (Other than FAI)?

At the moment, human brains are a cohesive whole, that optimizes for human values. We haven't yet succeeded in making the machines share our values, and the human brain is not designed for upgrading. The human brain can take knowledge from an external source and use it. External tools follow the calculator model. The human thinks about the big picture world, and realizes that as a mental subgoal of designing a bridge, they need to do some arithmetic. Instead of doing the arithmetic themselves, they pass the task on to the machine. In this circumstance, the human controls the big picture, the human understands what cognitive labor has been externalized and knows that it will help the humans goals.

If we have a system that a human can say "go and do whatever is most moral", that's FAI. If we have a calculator style system where humans specify the power output, weight, material use, radiation output ect of a fusion plant, and the AI tries to design a fusion plant meeting those specs, that's useful but not nearly as powerful as full ASI. Humans with calculator style AI could invent molecular nanotech without working out all the details, but they still need an Eric Drexler to spot the possibility.

In my model you can make a relativistic rocket, but you can't take a sparrow, and upgrade it into something that flies through space at 10% light speed and is still a sparrow. If your worried that relativistic rockets might spew dangerous levels of radiation, you can't make a safe spacecraft by taking a sparrow and upgrading it to fly at 10% c. (Well with enough R&D you could make a rocket that superficially resembles a sparrow. Deciding to upgrade a sparrow doesn't make the safety engineering any easier.)

Making something vastly smarter than a human is like making something far faster than a sparrow. Trying to strap really powerful turbojets to the sparrow and it crashes and burns. Try to attach a human brain to 100X human brain gradient decent and you get an out of control AI system with nonhuman goals. Human values are delicate. I agree that it is possible to carefully unravel what a human mind is thinking and what its goals are, and then upgrade it in a way that preserves those goals, but this requires a deep understanding of how the human mind works. Even granted mind uploading, it would still be easier to create a new mind largely from first principles. You might look at the human brain to figure out what those principles are, in the same way a plane designer looks at birds.

I see a vast space of all possible minds, some friendly, most not. Humans are a small dot in this space. We know that humans are usually friendly. We have no guarantees about what happens as you move away from humans. In fact we know that one small error can sometimes send a human totally mad. If we want to make something that we know is safe, we either need to copy that dot exactly, (ie normal biological reproduction, mind uploading) or we need something we can show to be safe for some other reason.

My point with the Ejypt metafor was that the sentence

Society continues as-is, but with posthuman capabilities.

is incoherent.

Try "the stock market continues as is, except with all life extinct"

Describing the modern world as "like a tribe of monkeys, except with post monkey capabilities" is either wrong or so vague to not tell you much.

At the point when the system (upgraded human, AI whatever you want to call it) is 99% silicon, a stray meteor hits the biological part. If the remaining 99% stays friendly, somewhere in this process you have solved FAI. I see no reason why aligning a 99% silicon being is easier that a 100% silicon being.