In some sense calling Arianism a "Germanic branch" doesn't make much sense, the doctrine didn't develop among Germanic peoples and it proliferated in the empire first, it became eventually an identity marker for a while so I guess if it survives it can take up the form and syncretize more freely with pre-Christian beliefs even more than Chalcedonian/Nicean Christianity did OTL in the region.
Obviously this concept could be extended, generally I don't think universalit religions tend to have outright clear cut "ethnic" denominations, just heterodox movements and doctrines that incidentally become ethnic markers.
I believe the key problem is that Arianism was relatively minor outside of the Germanic peoples and economics combined with the fact the Eastern Roman Empire was still alive and powerful meant conversion. It gave plenty of incentives to follow the Roman church rather than a schismatic branch. Maybe if the ERE gets mauled a little worse by the Huns, and Attila shares that greater success in the West, the Roman church would be damaged enough that those incentives wouldn't exist. Out of this, Arianism could overtake both paganism and the Roman church in Germanic lands, and from there we perhaps have Attila's Hunnic Empire reform itself (by avoiding the civil war through fortunate deaths, diplomacy, etc.) into something akin to the Frankish Empire, ruling from the Carpathians to the Atlantic, spreading Arian Christianity throughout Northern Europe.
If they had a state as powerful as the Frankish Empire and its successors in Germany backing it, they'd be forced to deal with them just like the Catholics were eventually forced to deal with the Protestants.
To have Arianianism prevail in non-Roman lands I think just having no major Nicean Germanic kingdom should be enough, the survival of the Byzantines concerns more whether or not Roman elites in Africa and Italy would convert alongside Germans, I seriously doubt it would influence what would happen all the way to Britain or even Northern Gaul in the long term, let alone Germania.
If the Germanic elites ruling over Roman lands(such as the OTL Goths, Burgundians and Neustrian Franks) are Arian then it would make no sense for transrhenanian people such as the Alemans, Bavarians and Saxons to convert to Chalcedonian Christianity, they would slowly convert to Arian Christianity and so would the common people.
