According to Wikipedia, there is no account of how the sphericity of the Earth was established. Though it goes on to say 'A plausible explanation is that it was "the experience of travellers that suggested such an explanation for the variation in the observable altitude and the change in the area of circumpolar stars, a change that was quite drastic between Greek settlements" around the eastern Mediterranean Sea, particularly those between the Nile Delta and the Crimea'
According to the same article the Sphericity of the Earth was widely accepted by the Greeks 5th century BCE. With Aristotle providing, what appears to me fairly flimsy arguments in 4th Century BCE, and more compelling evidence by Aristarchus in the 3rd BCE. This is quite a time-lapse, and hints at Aristarchus providing evidence for a theory already established. (A contemporary analogy would be Eddingtons expedition providing evidence for Einsteins theory of Gravitation - whose compelling physical idea was the equivalence of inertial & gravitational mass)
As the Greeks moved from a mytho-poetic cosmology to one focused on rational enquiry, were there general philosophical ideas that arose, and that would have proposed and made convincing the case for a spherical Earth?