If he wanted to, Augustus could create some system that gave him even less formal power, ...
Yes, and he already tinkered a bit: Think about the initial
consilium principis, Tiberius canceled or merged. The problem is, even if Augustus gives formally constitutional power to the senate or any other authority, he still has de facto the real power. And nobody can stop him or his heir to use it.
- he controls the army via his authority and the patrimonium
- he controls the treasury via the patrimonium caesaris
- he controls the senate via
adlectio and control of imperial careers
- he controls the plebs (comitia) via his authority, propaganda and
panem et circenses.
A state usually operates stably, if the constitutional power is with the real power. If this becomes unbalanced, it is time for constitutional reforms or you risk a coup or a revolution. If you look at an aristocratic republic like mid-age Venice, all power was with the aristocrats. A democracy would have had no chance. The aristocrats would had controlled the plebs and all institutions anyways. And they did their very best to avoid any autocracy / monarchy by balancing the power amongst the aristocrats and limiting the power of needed offices in terms of time and scope. Of course the aristocrats had the real power to do so. Sounds pretty similar to the roman republic before the 2nd Punic War, doesn't it?
In the roman world, autocracy had finally won. And the main instrument of power was the army. Even if Augustus never used his
imperium proconsulare and the legions to fight against his opponents in Rome, the threat was always there. Aristocratic republics had no huge professional standing armies. They used temporary mercenary armies or also temporary militia of the citizens. Therefore working aristocratic republics have always been rather small. They grew via coalitions e.g greek koinons. But this was limited, too.
The roman empire was not small and needed such a huge standing professional army. So the first question, we should answer is: How could an aristocratic republic control a huge standing army and the generals of the army without creating a new autocrat. How could Augustus abandon his
imperium proconsulare, without causing another civil war or at least way more usurpations than we saw in the next 250 years. Remember aside from 2 rather short civil wars after Nero and Commodus, we had not that many usurpations until 235 AD.
And, if you take away formal authority from the princeps and give it to somebody else, he has to get the real power, too. How do you do that? Is that possible at all? What is the real (informal) base of Augustus power?
And remember, this is just the 2nd and easier question. The 1st one was: Why should a true roman like Augustus expropriate himself and his family?