Nice post noname. You have earned yourself a bachelors degree in dominionology
I think I would recommend starting in the mid era. The game was designed from here, and the other eras were added in dom3. Thus the early and late eras deviate from the more basic balance of the mid era. Playing mid will give you basic knowledge of how the game and nations works, and less surprises from specialized nations. I think some of the descriptions on early and late nations refers to the mid era, so it might be thematically reasonable to test the mid era first. This is not true to all nations however. Agatha and the japanese inspired nations are new to dom3 and are probably thematically easier to follow from beginning to end.
Ulm was suggested as an opponent. Probably a good choice.
I think MA Man might be a good starter. Some diversification in human troops: longbows, medium infantry and knights. Some specialists to explore: bards, stealthy mages and wardens, elite knights. No unique national summons that steer research away from the basics. Air magic to test counters vs Ulm and offensive battle magic. Nature magic for some buffing, summoning.