Sounds like a cool project. You may already know about the Seshat project - https://seshatdatabank.info/ - which was started by Peter Turchin, Harvey Whitehouse, and Peter Francois in 2011 to collect a bunch of historical variables to study questions such as the one you pose about the Axial age.
They published an interesting paper that may be of interest on the subject. Mullins, Daniel Austin, Daniel Hoyer, Christina Collins, Thomas Currie, Kevin Feeney, Pieter François, Patrick E. Savage, Harvey Whitehouse, and Peter Turchin. “A Systematic Assessment of ‘Axial Age’ Proposals Using Global Comparative Historical Evidence.” American Sociological Review 83, no. 3 (June 1, 2018): 596–626. - https://journals.sagepub.com/doi/full/10.1177/0003122418772567
Sounds like a cool project. You may already know about the Seshat project - https://seshatdatabank.info/ - which was started by Peter Turchin, Harvey Whitehouse, and Peter Francois in 2011 to collect a bunch of historical variables to study questions such as the one you pose about the Axial age.
They published an interesting paper that may be of interest on the subject. Mullins, Daniel Austin, Daniel Hoyer, Christina Collins, Thomas Currie, Kevin Feeney, Pieter François, Patrick E. Savage, Harvey Whitehouse, and Peter Turchin. “A Systematic Assessment of ‘Axial Age’ Proposals Using Global Comparative Historical Evidence.” American Sociological Review 83, no. 3 (June 1, 2018): 596–626. - https://journals.sagepub.com/doi/full/10.1177/0003122418772567