A tyrant doesn't voluntarily devolve there power or retire and refuse to come back even after multiple people beg them to. If you're looking for a tyrant in that era I'd look to the golden boy Constantine. He was the one who seemed more motivated by his own self agrandizement then the well being of the empire.
Diocletian had his hits and misses but most of what he did was trying to help a floundering empire.
The tetrarchy didn't work in the long run but the third century had proven the idea of having one legitimate emperor just leads to civil wars. He overshot it and it turned out the natural state would end up being two emperors but he couldn't know that, and for someone traumatized by countless invasions from every direction all at once during the third century it seems like a pretty sound idea.
The price controls also are dumb in hindsight but for a society with no concept of inflation and it's workings it seems like a great idea to help out the people.
The reforms of the legions and the frontier strategy were sort of necessary at that point and were a large reason the empire maintained some sense of territorial integrity over the next two centuries.