After the Roman Empire collapsed the Germanic Tribes from Germania Magna invaded Roman territory and set up a bunch of Barbarian German kingdoms.
It didn't really happened this way, actually : most of the Barbarian kingdoms are issued from peoples present in Romania or at its borders since litteraly centuries (for instance, Franks settlement in Toxandria in the IVth century).
Meaning that they didn't so much invaded Romania, than progressivly replaced the roman imperium with their own : Barbarian institutions were largely a continuation of late Roman institutions on virtually all matters. It was helped by the fact various Barbarians, as peoples, didn't appeared before the IIIrd century trough Roman policies (Goths being, for exemple, a mix of Dacians, Sarmatians, Germans, Celts, Romans, etc.) that formed a distinct people because Rome needed an interlocutor.
By the Vth century, making the difference between a Barbarian and a provincial Roman isn't easy.
Eventually the map looked like this.
Actually, it didn't : this map is essentially issued from late XIXth conception on Barbarian institutions and evolution, and is subject to enormous criticism.
Syagrius' realm is an exemple on how it goes down to a huge speculation from sources without good historical reasons.
That said, you choose a bad period to illustrate the Romano-Barbarian kingdom extend : by the end of the Vth century, borders were still really porous and fluid, and while the WRE institution collapsed, you still had a large part of roman institutions in the west that weren't that tied up with Romano-Barbarian imperium (especially in mediterranean regions).
A slightly better departure point may be the early VIth, when the situation decanted : meaning Franks control most of Gaul, Goths most of Spain, Lombards most of Italy, etc.
Feel free to do whatever you want from keeping Christianity from existing to killing Clovis in his crib.
That wouldn't do any good : even if Clovis doesn't convert, western Europe was at this point hugely rooting for Chalcedonism. In order to strengthen its rule, a Barbarian ruler would have little choice than to abide by Christianism (would it be only because he was the continuation of Roman imperium) and if he's not Homean as Goths were, well it was bound to a Chalcedonian baptism.
If not Clovis, then anyone present in Northern Gaul would do.
But I wan't the borders of the Germanic kingdoms to last into the present day.
That's not possible : no border last for 1500 years untouched.
That said, if we have more reasonable objectives, such as having these borders serving as the basis of modern-day borders rather than medieval ones (for western Europe obviously, because it's definitely not the case elsewhere), you'd need to avoid the institutional and political rupture of the VIIth century.
Because of the Romano-Persian, then Romano-Arab wars, the connection between ERE and the western kingdoms declined, at the point they ceased to form a same post-classical cultural and political ensemble. Before that, well, Romano-Barbarian kingdoms were fairly in the continuation of the late empire and their borders usually followed the old diocesan or provincial limits.
Getting rid of the Arab expension may be a huge help to avoid the decline of dynasties such as Merovingians, or the anti-dynastic Gothia.
You'd still have changes, such as North Sea basin blossoming, but it wouldn't be as radical, while the decline of late Roman institutions in the west was clearly unavoidable. But you may see something replacing it less conflicting as carolingian feudality was.
Geopolitically, it could translate as having late Roman concepts being transformed, and old regional ensemble being seen as a natural political "horizon". Note that for centuries IOTL, up to the XIIIth century in some respects, it was the case. But there, it might have more of a chance.
But again : it would be an historical basis, not something lasting one millenium and half.
As a bonus, because I'm a proud descendant of Allemanic Germans, make it where only the kingdom of Allamania survives.
Actually, you didn't have a Kingdom of Alemania at this point. Not only it's suspected that it goes down to a typo (Clovis fighting Alans instead of Alamans), but Alemani themselves may have been politically closer to what Saxons were later in the continent : a bunch of chiefdoms more or less tied but without a clear overlordship.
Gibuld might have been, as Windukin after him, "merely" a chief managing to get the lead of a loose defensive confederation.
The first clearly definied and institutionalised Alemmania is, ironically, a Frankish construction.