Democracy is Dying Well, some quibbles, I suppose.
The Etruscans were not the great power in Italy, and indeed were never so far as we can tell united into a single "power" as we would understand it. They were a rather loosely organized group of cities in central to northern (not southern) Italy (thus, "Tuscany" in northwestern Italy derives its name from "Etruscan" or "Etruia). To the south were the Greek colonies of Magna Graecia. The Greeks held most of southern Italy and Sicily during the time the Etruscans were significant actors in Italy. Greeks, Carthaginians and Etruscans jockeyed for position in Italy, alternately at peace or at war while they traded and otherwise interacted with each other, in the years before Roman dominance. Even the Gauls had their hands on portions of the peninsula. The final Etruscan king of Rome, Tarquinius Superbus, died about 350 years prior to 150 B.C.E.
The last war of Rome against Carthage began in 149 B.C.E., so it's not really correct to say Carthage was defeated in the 150s. I'm not sure who you mean by "populist strongmen" but the Gracchi brothers didn't begin their efforts at land reform until around 130, and that didn't work out very well, brother Tiberius being beaten to death in 133. The struggle between Marius (a new man, but not I think a populist strongman) and Sulla, and Sulla's dictatorship, probably set the stage for the end of the Republic. Most of the big actors in that end, Caesar, Pompey, Crassus, began their careers as friends or foes of Sulla, that remarkable and very dangerous man. I think Sulla's dictatorship was the trigger, though somewhat oddly as he actually retired from power to live out his days in what we would now call "partying." He showed what someone with unlimited power could do. He led his legions into the city, the firs time any general had done so, seized power and kept it as long as he liked. He was Caesar's precursor, and paved the way for the Principate.
I think we have to be careful in comparing the U.S. to Rome. There's no question the founding fathers admired it and drew inspiration from it, but Republican Rome was significantly different than we are, even now. The Principate was established and sustained by control of the military.