. . . . Using the Republic effectively means you have to take the time to perfect everything, which mean you probably won't declare war until the industrial age. You'd have to work every tile, get every tech, and build every improvement you can before you declare war. . . . .
Not really. I use Republic constantly. I've never made it to the Middle Ages without war. Sure, you should work every tile, but that's true regardless of your government.
But, yes:
Republic does require paying a little extra attention to your cities, but shouldn't you be paying attention to them, anyways?And using it properly means managing your cities every other turn.
For Monarchy, all you need are shields and money, and the best damn unit in your time period. Wars start in the Ancient Age, and by the time the game ends, every civ is so hindered in tech that you only reach the early industrial age.
Monarchy doesn't provide any more shields than Republic does, and it provides less money. The additional commerce speeds research and helps you pay for unit upgrades. And I'm not sure I understand why it'd be a bonus to have "every civ [. . . ] civ so hindered in tech that you only reach the early industrial age." To my mind, the question is not whether I can slow the whole world's tech pace down, only whether I can keep up.