I agree : when I said Northern Germanic influence on Barbarian kingdom warfare, I was mostly thinking of the influence you already had in Late Roman army, as well as (but that's mostly concerining Frankish and Anglo-Saxon warfare) the backleash of Roman collapse in Barbaricum, as pointed by Ingrid Ystgaard.The Angon seems an awful lot like a northern European take on the Pila, which is more support for the idea that continental early medieval tactics were loosely based on Roman tactics.
Roughly, trough the destabilization and pauperisation of Scandinavian polities, with a wider use of axes as hand-weapons.
It's still pretty much goes down to Roman influence, but I prefer to distinguish it from the direct Roman influence on weaponry and tactics that romanized Barbarians went trough.
If you'd allow me, I'd advise you Merovingian Military Organisation, by Bernard Bachrach.I love the idea of throwing the Angon into an opponent's shield then forcing the haft down to expose them described by Agathius of Myrna.
I'd try to hold up the digression from the topic there now, as far as I can