Disguised as (human-sized) nonhuman animals
A human brain may be hard to beat energy-wise: <PERSON>'s principle limits the efficiency of computers. Biology doesn't have nanotech, it is nanotech. No technology can make atoms any smaller. Thus the human brain may be basically as small as it can be without sacrificing intelligence.
So lets live in the amazon as Jaguars. 50 years after we put human neurons in a mouse brain we can transplant a whole human brain in a Jaguar body (its a bit of a tight squeeze in that skull but much more realistic than insect-size).
Small-tech
A large industrialized factory is so (early) 21st centaury. What does a society need?
1. Food: Jaguars can do that at Jaguar-level intelligence. High-tech human-brained Jaguars will have no trouble staying fed. Wear hidden AI cameras and the Capybaras are toast.
2. Safety: It's hard to say how much of the Amazon will remain and what poachers will do. Thankfully, poacher tech is designed to catch Jaguars, not cyborg-enhanced Jaguars with passive radar and other detection methods. Artificial pelts and lab-grown meat are much easier anyway.
3. 335
Opportunity: 2070 will have such exquisite virtual worlds a society can live a life of computers and tech. For work as well as play: you can design everything virtually with 2050-level physics engines. There is "no" need to build massive infrastructure until all your "take-over-the-world" blueprints are ready.
But my computers? Thankfully, photolithography reached the "well-funded garage" level in 2020, so by 2070 a basic fab can be carried in a backpack. Feedstock materials and spare parts are a problem, but they can be (laboriously) prepared with furnaces and other low-tech methods. It's far less efficient but its enough to barely self-sustain a tech industry. Moores law died around 2025 so being "two decades behind" isn't much of a loss, even with the underclocking and material choice you need to keep the chips alive for ~20 years.
Don't blow your disguise!
Humans in 2070 are aware that mind-downloading to animals is possible but it is rarely done: it is easier to replace organs or even whole bodies. And most who do so aren't in a secret society, they are just weird people in our society being weird.
Although humans are not eagle-eyed for animals with human brains, they will begin to get suspicious with enough odd behavior, more so those who know how animals are supposed to behave. Humanized Jaguars have a natural quadruped gait, but spacing out into a virtual world is not really a <PERSON> thing. Also your paws look strange. Most items are carried inside the mouth or swallowed to look innocent, with larger items assembled like a well-machined jigsaw puzzle at destination. Furnaces and other infrastructure is disguised as a disorganized rock pile (and placed at the base of cliffs where rocks fall into talus piles anyway), etc. However, a close inspection by a human would reveal what it is. So stay safe, stay hidden! 337