I would agree with first part of your posting but not the last.
Indeed, the beginning of the end was when the Army(or was it the Praetorian Guard?) over threw and murdered Emperor Pertinax after the assassination of Commodus and sold the office to the highest bidder.
Then the army of Septimus Severus came in and established his reign. His cynical reminder to his son, “make the soldiers rich, care little else about the rest". ---------------------------------------------------------------------------------------- As for emperors who tried to give power back to the Senate, I cannot think of any. Not Augustus, not Aurelius........?
Tiberius tried to give some back and there were a couple of others. Read Caesar and Christ by Will Durant.
I meant when then the army started appointing the emperor, such as Titus and those before him.
Marcus Aureleus?
Julian II the Apostate also made moves to decentralise power back to the cities and the provinces.....