You're right that eastern Rhineland weren't comparable to Gallic Rhineland, altough the limes was fairly prosperous due to its role in Barbarian and Northern Roman trade. Initially, Salian Franks were settled in Toxandria, in modern Netherlands, while other Franks were settled either as foederati or as dedicitii along both banks of the Middle and Lower Rhine. As Clovis conquered most of Gaul, he also took care doing the same on Rhineish Franks and repealing back Alamans.

His sons( partcularily Clothar and Theuderic) took over Alemanic and Thuringian peoples, while attempting to put Saxons into their influence (it's hard to establish how much Saxons were Frankish tributaries but at the very least, they fought a lot against them afterwards), as Ostrogoths were unable to support these as clients.

Rhine definitely ceased to be a frontieer but remained a main trade way between Mediterranean Sea and Northern Europe, Frisia becoming more and more a major trade node to the point Frisian became synonymous with sea marchant in the VIth onwards. And altough Alamans, Thuringians and eventually Bavarians were often let to their own entities, with Frankish or semi-Frankish nobility, they (as well as Saxons) gave an important tribute when Franks could pressure them, pointing the region wasn't exactly desolated.
That said, while Franks were present in eastern Rhineland, they weren't there in the same numbers or significance than in Gaul : places as Frankfurt were probably already existing in the VIIth century, but western Germania was still largely unerdevelloped compared to former Romania, an a good part of the prosperous part were along the Rhine, or were taken as tribute or raiding loot.

It didn't changed significantly during the decline of Merovingian dynasty, except that Germanic peoples gained more and more independence, but eastern Frankish aristocratic families began to assert their dominance and to expand their lands eastwards.

Carolingians, an aristocratic family of Austrasia, the eastern part of Francia, took over and were bent on affirming Frankish authority over these peripheral territories : Frisians, Alamans, Bavarians, Thuringians and eventually Saxons were brutally submitted to Frankish rule, with a wave of "Frankicisation" of native elites trough baptism, palatisation an settlement especially west of Weser (a rough limit of Frankish authority in the VIth to VIIIth centuries) : Fulda, Paderborn, etc. almost always as much missionary seats than market places "gridding" the territory (called Franconia, as the country of Franks, but the same could be said about Saxony), with a significant rural development still discernable in medieval or even early moden periods to have been initiated in the IXth century.

It had been argued that the settlement effort by Carolingians, and then Ottonians in the Xth, was comparable to what happened in the same time in Spain, with a determined nobility (which benefited from the particularily damaging political mosaicisation that happened in Western Francia an Aquitaine), in spite of the Viking raids of the IXth/Xth.

The combination of a strong royal power in Germany, of a lesser political desintegration, of a constant manorial mindset in the Xth from both nobility and monastic or episcopalian establishment

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