following example provided by Diocletian/Constantine/Justinian ?
- autocratic rule
- large bureaucracy to collect taxes
- religion as unifying ideology
- creation of large mobile central army / comitatus
- establish frontier army at Danube frontier and Armenian frontier using theme model
- strong navy
- capital at Byzantium area
- set semi-independence and hereditary rule for Britannia, Africa Numidia, Hispania, and Gallia. (as reward for general)
- maintain client states in Nubia, Arabia, and Armenia
- priority on diplomacy with Persia
Byzantium had most of these and was still quite limp wrist when facing only minor Islamic resistance and then defeated fairly easily by the Saljuq... Having a large bureaucracy to collect taxes means what exactly? Byzantium had this and all it created was resentment towards their rule and created a class of people who tended to vie for their own interests. Autocracy may be a positive in some cases, it depends, once more, Byzantium had this otl and its period of ascendancy lasted a relatively short period and the conquests were minor for what one might expect.
Religion as a unifying identity is positive, but which religion? Perhaps asserting a deification of the monarch or so forth? Strong navy is probably fine, but what would be its use to have a large standing navy paid by the state? The government can simply promote a merchant-marine that it does not need to maintain, similar to many successful ancient empires preceding Rome, such as Assyria, Achaemenids, etc...
It is difficult to maintain fair relations with the Sassanids and yet control Armenia. The Sassanids cannot simply allow Rome to protect the Arsacid successor state in Armenia. Rome if they are to protect Armenia, must make a decision:
1. Forget Armenia, let the Sassanids have it and remove the Arsacids, traditionally Roman enemies. This will allow a detente and fair relations with the Sassanids for a time.
2. Use Armenia to restore them to the throne and create a pseudo-Roman puppet in Cteshipon. In otl, this is partly what Emperor Maurice did, he utilized the Armenian nobility with his own armies to oust Bahram Mihran and restore Khosrow II upon the throne.
Ultimately, the second option may be the best for the long term prosperity of the east. This also could garner massive political fortune to the emperor who succeeds in this campaign.