Would reforming legions (and administration) as Diocletian/Constantine did, help? I mean- increase taxation, levy more soldiers, fortify border (but after establishing it much further east). Was it possible at all at this point? Or maybe something a bit less drastic, increasing Roman military temporarily, just to allow permanent pacification of this region and securing eastern border? I admit that it seems like costly endeavor, but really worth it in the long run- even excluding possibility of Sassanian rise.
I am convinced, that some of Diocletians measures, especially his new tax and supply system rescued the desolate roman empire. But it also ruined the empire longterm, because this system of
annonae based on
capitatio et iugatio could easily lead to a detrimental overtaxation. Especially in combination with the militarisation of the civil administration and the society introduced by Diocletian.
The military reform, which led to the more flexible late roman army was not Diocletians invention. It was rather a gradual process beginning with Gallienus (if not earlier) until the sons of Constantinus. Nevertheless, adaption of the army structure to the enemies strength always help. And the romans adapted always. Just look to Arrians report about the battle against the Alans and you see how flexible the roman army of the 2nd century always was, if facing a cavalry army.
The romans could have used the great income from the Dacian wars in order to get the legions needed. These are about 12 as mentioned above. Some of them not new legions, because you could use some (not all) of the 6 legions in Syria and Cappadocia. After Media is conquered, the iranian deserts are the new border, and a client King in Persis and Parthia is established, the number of legions could be slightly reduced. Just slightly, because you need forces to secure the hinterland. However the main forces are now in Media and Armenia to secure the new eastern border. The additional ongoing costs for more legions from now on could be covered by the additional taxes from the rich new provinces.
But as mentioned above: Military was not the main issue! Thats the much easier part of this adventure.
If you deploy just 5 legions to Media, you are in trouble, if you deploy 10, you get more trouble, and with 15 trouble would go thru the roof.