As I already proposed above, a provincialisation of Armenia and Mesopotamia (except Characene) might be sufficient. Of course the Zagros-Mountains are a lousy border from a military point of view. Therefore you need buffer-states up to the iranian deserts, which could be Media Adiabadene, Media Atropatene, Susiana and Characene. It is also a good idea to control the King of Persis. Best case the King of Parthia, too. But that might not work that good, because he is far away.
For such a scenario you need at least one campaign up to Persis and Parthia and you have to beat them all, or at least they have to accept the roman emperor as their new King of Kings.
The key is to avoid, that they ever unite again under a King of Kings. Even if just 2 of them form a coalition against Rome, the empire might be in trouble. But with legions nearby in Mesopotamia and Armenia, control should be possible. Some TLs did something similar, but they implemented just one parthian client-king for all these regions east of Mesopotamia. I doubt this would work. Mesopotamia is the richest province of the parthian empire, but that does not mean, that the rest has not the power in terms of economy and population to strike back. You have to desintegrate the parthian empire, by cutting it into small independent pieces. At least near your new border.
Another option is a border at the iranian deserts with a provincialization of Media, and perhaps Persis. From a military point of view this is the far better border. My problem is, that as more eastern provinces you create far away from Rome, as more you run into a cultural and political nightmare. Already Mesopotamia could mean, that you overstretch the empire. Not from a military point of view. You overstretch the capabilities of an ancient centralistic governement. And centralism is one key-success-factor of the roman empire.
PS: I forgot the Caucasus north of Armenia. Of course another three client Kings: Albania, Iberia and Colchis.