“Augustus self-declared,...”
Not really. He NEVER called himself “Emperor” iirc.
The historian Werner Eck states:
The sum of his power derived first of all from various powers of office delegated to him by the Senate and people, secondly from his immense private fortune, and thirdly from numerous patron-client relationships he established with individuals and groups throughout the Empire. All of them taken together formed the basis of his auctoritas, which he himself emphasized as the foundation of his political actions.
I see Augustus was kicking back in the Palace and Livia said, hey, good news the senate just made you emperor. He didn’t want to be associated with anything like being a king. He just wanted the power.
You may also believe that all legislation starts in the congress because the constitution says so.
I apologize for being so literal. He didn’t run around calling himself emperor but he made al the power and political moves to be declared so.
Here’s more information:
By the year 13, Augustus boasted 21 occasions where his troops proclaimed “imperator” as his title after a successful battle.[157] This is well after the word was used only as a name for victorius general.
/bingo
Pompey was given the title, basically an honorarium, as were others before and after. The Roman republic, such as it was, didn’t evolve into a monarchy until the 3rd century.
The office created for/by Augustus was merely an evolution of the Roman system, which had long before begun to need an executive branch. The three dozen or so families of landed gentry who had been inheriting things, and running things for centuries for their own benefit, fought tooth and nail against reforms of any kind, continuing to try to enrich themselves through imperial expansion, confiscation, and slavery.