This has been my first look at this timeline, so I will restrict my comments to this latest section.
Its quite interesting that you have had the Sassanians collapse in the early 4th century. What was your rationale for this? Will you have the Palmyrene Empire seize some of its western satrapies while Ctesiphon is busy in the East?
Secondly, its also interesting that you have had the cult of Sol Invictus become officially established as the religion of the Western Empire. Has the traditional pantheon been suppressed, or does the cult co-exist with it, as seems more probable?
Firstly, the Sassanids' collapse was due to a number factors, any one of which by themselves probably wouldn't have harmed the Empire, but which together proved too much - Narseh's attempted coup, the Lakhmid conquest of Mesopotamia, Bahram III's attempt to bring the Arab tribes to heel, the destruction of the Kushanshahs. That said, the Sassanids aren't completely gone. Bagra sees the advantage of having Bahram IV as an easily ruled puppet, so he installed the boy-shah as ruler of a territory roughly corresponding to Media Atropatene (which is what I'll call it if no one suggests a more realistic name). Bagra keeps Bahram flooded with concubines, wild animals, minstrels, and any other form of leisure he wants. It's possible the Sassanids could use Atropatene to rebuild their empire if the Afrigids prove unlucky, but it's unlikely to happen within Bahram's lifetime.
Secondly, the Sol Invictines have largely adopted dieties which were popular in the western part of the empire - Mars, Mithras, Isis, some of the Celtic gods, etc. The old Capitoline faith had largely stagnated by the time of the POD. It still exists, but has largely been sidelined by the newer religions in the former empire.
What's the current political, cultural and religious situation of Dacia, Britain and near the Rhine and Danube?
Britain is ruled by Romanized British noblemen, the ancestors of OTL's Romano-Briton kings. Thus far, there have been little changes from OTL. The Romano-Britons stil live semi-independently of the continent, and Christians live easier here than in Europe. The Sol Invictine faith is light in Britain, with the primary religion being a Capitoline-Druidic syncretic faith.
As for the Rhine, it is a highly militarized zone meant to defend Gaul from tribes like the Rugii and Burgundii. Dacia and the Danube have longsince been surrendered to the German tibes (and, TTL, there is no differentiation between "German" and "Germanic". Make of that what you will).