I agree, turning points for Rome'd downfall were the adoption of greek, degenerate customs, which caused the civil wars of 1st century bc, and the citizenship granted to foreigners by Caracalla.
Roman strenght, as Cato remarked often, came from the fact that the society was very much traditional, based on community duty and not seeking personal wealth or hedonistic pleasure.
This is what allowed them to become strong and expand, also, I remember you, by granting only a "social citizenship", not proper real one, to subjects outside of latium.
As soon as carthage was destroyed, romans did not have enemies, they became wealthy, and started to look at foreign customs, like greek, which true roman people like Cato considered disgusting and degenerate. This different mind set brought the problem of social differences between plebs and patricians, the gracchi reforms, the civil war between marius and sulla, caesar and pompey, and octavian and mark anthony in seemingless succession. Ultimately, it lead to the empire, which with time became more and more degenerate and represents the root itself of the roman demise.
As a matter of fact we are repeating history, we are in the degenerate undecent part of roman history, and eventually I am conviced this will lead to the demise of western society and the possibility of starting from scratch once again