I'm thinking the High Thonians were descended from refugees of Kelnore, but common Thonians are more closely related to the Tharbrians, with some eastern Tangor admixture in the south.

If the Great Kingdom has parallels with Greyhawk's history, the Lhomarrians would be like the Suloise, the Tharbrians like the Oeridians, the eastern Tangor like the Flan, and the Gishmesh like the Baklunish.

The War of the Pious and Philosophers acts like a restart button for the Wilderlands setting, with every culture reduced to barbarism and having to begin again from the ground up (like the Twin Cataclysms/Great Migrations of the Greyhawk setting), so anything prior to about BC 10,000 in the Thyatian calendar would be largely speculative/vague/irrelevant to most of the Blackmoor-era cultures anyway.