Actually this is a reason why it should be easier to butterfly Christianity's ascent away.
If I make you a criticism, it seems like you think Germania is going to be way it was in 1st century CE for ever. The economic, cultural, and demographic effects of Romanization (especially with the early discovery of hevy plough and horse collar) shall be compelling within a few decades and overwhelming within a few centuries. Roman and its successor states never really did any effort to settle Germania between 1st and 8th centuries, so what happened since 800s and 1200s IOTL shall happen between 1st and 5th century ITTL, only faster and more effective since it not shall not by done by disorganized feudalism, but by a centralized state that can build roads, canals, cities, colonies, etc. In the long term, (say after 50-100 years), Germania Magna shall become just as economically-demographically developed and loyal to Rome as northern Gallia and shall not need not need garrisons to keep it pacified any more than Gallia, Iberia, or Britannia. Only the borders on the Vistula shall need heavy garrisoning, but that shall be one of the two main taks of the army.
I can see your point in Christianity, what I'm saying is that explain the rise of Christianity as a response to the 5th century crisis alone (people becoming Christians because it gave them a sense of peace in a crumbling world) it can be a little simplistic. I´m not expert in Christianity so maybe someone who is can offer a more in-depth vision.
What I did mean about Christianity is that you have by the third or forth century a sizeable community, therefore it should be added to the list I commented.
About Germania (and Persia) what I'am saying is that both are huge, and both offer too many challenges to absortion, not that it´s impossible, only that is more hard that you seem to assume.
I would say, that you would have to pick one and resign to lose the other. You should work a timeline for the sake of clarity (and arguing).
You talk about the demographic, economic and cultural effects of Romanization, but romanization was a long long process that spared for centuries, not decades. What would be the population rate of growth in pre-sanitation Germania? 0.1% - 0.2 % ? Economic growth it can't be that high either, you don´t need to conquest Germania to secure a source of amber, and other commodities where available in Britannia, northern Gaul or places nearer to the imperial costumer's core, and Dacia has plenty ore and lead to offer aswell. And about cultural romanization, well it´s complex. Language and culture of 6th century Carthage was Punic, not Latin (7 hundred years after conquest), and even in Gaul, language of the masses was Gaulish by the time of the Franks, same goes in Hispania (Basque is still there) and in Britania didn´t prevail (besides some few words in Welsh), not to mention Noricum or Panonia. Then, it´s a long and frigile process.
And finally about loyalty:
there were legions in Hispania, Gaul and Britannia. And rebellions sparked in Gaul and elsewere (seldomly?). If you want to add several new provinces to empire, my guest is that you will have to add several legions aswell to maintaining them loyal to Empire, then your new Vistula line will not necessaryly cut costs (or legions) to deploy elsewhere. At least not instantly at least not until roads, channels, cities and so are actually built and florish.
Persia, as it was said, is too far, too big, too populated and too hard to assimilate to be integrated into the Empire.
So again, IMHO, it´s Germania or Persia, and i would advice Germania.