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The solution is to give the MANY Romans or Romanised people this sense of belonging, participation and duty back.
I must agree. What I have in mind is not comparable to settling Franks. Why settle barbarians when there are so many people within the Empire? People say "the barbarians were martial". What does that really mean? If not this: too many Romans were so detached from their political structures that they couldn't be arsed to lift a finger for their defense (because exclusion was an ill-guided policy, and because the Roman imperial state hovered absurdly somewhere above the many regions and cities, being nobody's particular lovechild), while thid was not yet entirely the case with the Frankish tribes.The themes were administrative and military divisions, comparable to the provinces of the principate. The strategos of a theme was, like a governor of the Roman republic, both administrator and commander of the army within this province.
Sure the execution of the laws was devolved to the themes, as before to the provinces, which were civil during Late antiquity and both civil and military during the Republic and the Principate.
But in both cases you settle hostile armed forces within the Empire and hope they will honor instead of stabbing you in the back at the first opportunity.
Divide large barbarian tribes in little groups, mix these units with groups from other tribes, send them into different parts of the Empire; settle them down as soldier peasants on the model of Byzantine thems; that would be reasonable.
The solution is to give the MANY Romans or Romanised people this sense of belonging, participation and duty back.