Just gave it a quick read through.
A major problem is population which in Scandinavia was quite small. Another changing weather patterns so havests would fluctuate back an forth mainly during the Iron age which is precisely when this is going to grow into a major empire.
Southern Scandinavia was petty statelets warring among each other till Kingdoms emerged in Denmark around 700 somewhat later in the other two lands.
Even when the Vikings were going amok in Britain they had to some extend rely on rebellious locals to swell their numbers.
Later the Danish kings of the 12. century would do the same thing in the Baltic area.
Scandinavia just couldn't field the population needed to counter something like the Roman Empire.
In the timeframe of the OP they were just too happy infighting or going Cuchulainn style rampaging on their neighbours.
The only thing close to the wishes is Chochilaicus' and possibly others raids on Friesland around 516 as recounted by Gregory of Tours.
But then the Scandinavians have to discover (this isn't CIV but anyway

) sailing - been rowing till now - which they funny around to do till about 730 or some such.