Unfortunately, Asimov never had the chance to publish a description of Galaxia. He did write extensively about the planet Gaia and the telepathic humans who lived there. I don't think that we can safely assume that Asimov intended Galaxia to simply be produced by converting all 25,000,000 human worlds into copies of Gaia. If Galaxia is not just a network of 25 million Gaia-like planets, then what is its structure?
Also, Asimov never had a chance to show us the adventures of Fallom after receiving R. Daneel Olivaw's memories. I think we have to ask why it was the case that Asimov showed Daneel deciding to transfer his mind into Fallom's biological brain. Did R. Gohrlay and Daneel have the same motivations for ending their existence as positronic robots? Exactly what was the relationship between R. Gohrlay and Daneel?
In Foundation and Earth, Asimov showed Daneel making the claim that he was dying. Having said that, and having said that it is possible for Daneel to transfer his mind into a human brain, we need to ask why he selected Fallom's brain. Unfortunately, at the point where it becomes clear that Daneel wants Fallom's brain, Daneel falls silent and we learn nothing more of his plans. Asimov did provide us with the thoughts of Trevize about Daneel's motivations: Trevize suggested that Daneel would need Fallom's "transducer lobes" and that Fallom's long life span would be useful.
One of the major gaps in our knowledge is how Asimov viewed the relationship between "transduction" and telepathy. Asimov had Bander, a Solarian with "transducer lobes", explain that those brain regions could collect useful energy from any available heat gradient and channel the collected energy though the lobes to machinery and light fixtures which were then powered by that energy. I imagine that there is a new form of energy that must be involved in "transduction" and I imagine that it is related to the means by which telepathic signals are produced and exchanged.
The Solarians are a unique and highly specialized off-shoot of humanity. There are only about 1,200 Solarians, but they make use of a small army of robotic helpers. Humans in the rest of the galaxy do not make use of robots. The Solarians prize long lives and they have been genetically engineered to live long and make use of their transducer lobes to control their robots. Each Solarian is very independent and protective of individual freedom. In contrast, Asimov depicted telepathic Gaians as happy to exist as part of a group consciousness. With the exception of Solarians, all humans in the galaxy have life spans similar to Earth humans in the 1940s.
For The Start of Eternity, I assume that Daneel was the creation and tool of R. Gohrlay. R. Gohrlay wanted to remain within the protective cocoon of of her time travel device on the Moon. She needed a telepathic robot who could exist in Time and work (secretly) to bring Galaxia into existence. Asimov described the development of Galaxia as a project that involved about 20,000 years to perfect Gaia and that would require a few more centuries to convert the 25 million human worlds of the galaxy into Galaxia. The Gaia project was very secretive and involved the development of telepathic humans. Similarly, the telepathic humans of the Second Foundation remained secret until drawn into the open by the Mule. I assume that Daneel was able to keep the existence of telepathy secret from the Huaoshy until about that time. Daneel faced some vexing technical problems for the establishment of Galaxia and had to create the Foundation as a way to allow a large group of humans to study "mentalics". That study was begun by Dr. Darell and completed within about 100 years.
I assume that one of the key technological hurdles for Galaxia was a way to make it immune to attack by the kind of "mind static" weapon that was first developed by Dr. Darell for use against Second Foundationers. I assume that positronic brains remained sensitive to some kind of attack, either "mind static" or the type of attack used against Dors Venabili. I assume that the best defense for a telepathic individual was to have strong telepathic abilities similar to those of Daneel combined with a powerful "transducer lobes" in a human brain, so R. Gohrlay would have had Daneel send Trevize and Bliss to Solaria to capture a Solarian.
Rather than convert all 25 million human worlds to copies of Gaia, I'm thinking that most human worlds would have served as a kind of "sensory net" for Galaxia, sensitive to invading Huaoshy agents, but with only minimal telepathic ability. For example, the humans of Tokishira as depicted in "Welcome" to Galaxia are very similar to Foundationers, but have some unconscious telepthic abilities that allow them to function as part of Galaxia. Gaia and a few other secret worlds would function as the "executive committee" of the Galaxia group mind.
What would be the fate of the Foundationers as Galaxia comes to dominate the galaxy? I assume that the Second Foundationers could have been re-oriented towards the task of spreading humanity beyond the Milky Way galaxy. The First Foundation scientists would also have directed there efforts towards human expansion to other galaxies. Daneel, perhaps with a "streamlined" positronic brain would have remained as a kind of "decoy" for attracting the attention of Huaoshy agents and continued working with the Foundations, as indicated in the chapter about Daneel's visit to Terminus.