Frankish hegemony over transrhenan peoples was less due to conquest or overpowering campaigns, and more to a policy of personal relationships, trade, raids and counter-raids that weren't that dissimilar to Rome's policies in Germania and that tended to define sort of a Frankish "sphere of influence/power projection" up to the Elbe, but also in Northern Italy, Armorica, southern England and Spain.

While Clovis battled against Alamans and Thuringians already in the late Vth and early VIth centuries, it mostly concerned groups established or raiding Gaul, not campaigns going beyond the Rhine. These peoples entering into Ostrogothic protection under Theodoric, the establishment of a Frankish hegemony in Germania can be more easily associated with the reign of Theudeuric I, who ruled the Northern-Eastern part of the Frankish realm and directly at contact with its polities (or raiders, as evidenced by the failed Danish raid of 516, whom defeat made enough of an impression to be accounted for in Beowulf).

This hegemony can mostly be traced from the reigns of Clovis' sons, especially Theudeuric who ruled over the lands associated with Franks since the IVth century and that was in direct contact with Frisians, Saxons, Alamans and Thuringians. The last two peoples already clashed with Franks during Clovis' reign, but mostly groups that were present in Gaul, the rest benefiting from the powerful and prestigious protection of Theodoric, king of Ostrogoths, a protection that disappeared in the VIth century along the decline and fall of the Ostrogothic kingdom itself.

As Francia appeared as the most prosperous and powerful polity of post-imperial western Europe and as kings in Italy were unable to really preserve Theodoric's diplomatic network, Franks were a necessary and powerful partner : the first Frankish intervention in Thuringia (with Merovingian kings probably had genealogical relations with) was even made at the behalf of a Thuringian king against another, hoping to get half of it which they did after an obscure situation where the supported candidate died. It's not that they annexed all of Thuringia : a good part was eventually swallowed up by neighboring entities, Germanic or Slavic, and the land within the realm was probably let to local nobles (led by a duke at least in the VIIth century, hinting at an "ethnic" rulership even if he was Frankish), poorly settled at best by Franks, with local populations being held tributary. The Thuringian exemple, eventually, can be set for the lot of Germanic principalties under Frankish influence.

Rather than a conquest, what was important for Merovingians was their capacity to halt raids, raid beyond the Rhine themselves and obtain both loot and substential tribute, raising auxiliaries and enforce their claims of over-lordship by including them into a personal and genealogical relationship as duces (dukes) although they probably had royal titles (in a system not unlike the Chinese tributary system, local and regional kings in Germany, Wasconia or Brittany weren't considered as such by Franks, who called them counts or dukes). On this regard, the difference usually made between Alamans, Bavarians and Thuringians from one hand; Frisians and Saxons from another one might not be that radical (Saxons, for exemple, being extorted a tribute and considered as rebels when refusing to pay up).

Of course, this was true when the Merovingian kings were able to enforce their rule beyond their borders, bullying their way into submission of local kings (especially under the reigns of Theudeuric I, Clothar I, Clothar II and Dagobert); but at the first sign of weakness, a revolt was always possible as it happened to Clothar I against Saxons (and as Marcomanni did with Marcus Aurelius in their time); even the own successes of peripheral rulers could led them to challenge Franks (such as Radulf, victorious against Wendish raiders and successfully beating Dagobert's Franks).

This could give the impression that Frankish Germania was kind of an aftertought, but it seems to have been rather the contrary : tributes payed in cattle, horses, possibly slaves were important, levied men served as auxiliaries in Frankish campaigns as soon as in Northern Italy and as late as the VIIIth century. It's just that, as Romans before, Merovingians were content (or had to do) with a fluctuating and warlord-ish relationship where their hegemony had to be regularly reasserted by demonstrations of strength or battle; rather than an effective conquest. That said, even this complex relationship left marks : local dynasties and nobilities were "Frenchified" to an extent, allowing Carolingians to maintain genealogic ties with dynasties such as Agilofings in Bavaria, giving some leeway to integrate them further to Francia (establishing law codes, notably, and "preparing" their annexation) and serving as model for further conquests such as in Frisia or Saxony : these conquest, as real they were, were also not so much "efficient" than brutal and requiring a lot of resources and attention from early Carolingian kings compared to the more light weighted management of Merovingians.