There was some settlement in the Loire Valley before the Saxons were pushed out in the early 460s.
And these settlements are quite comparable to what Norses did in the IX, fortified outposts rather than real takeover of the lands, at least not willingly (The defeat of Saxons against Frankish kings may have led these latters to deport Saxons in countryside, like they did, in larger scale, with Saxons in VIII/IX century)
Belle-Île, by exemple, may likely have been used in the same way Noirmoutiers was later, as a advanced base both for raiding and for trade (Saxons, understood as the merging of coastal peoples that may have been as well Frankish, Frisians than actual proper Saxons, controlling the naval economical exchanges).
Now, while the use of "Saxons" for naming part of Bessin, may refers more to Saxons deported by Carolingians rather than a continued "maritime" Saxon settlement, they did maintained many particularities up to the VII century : particularly by being a military force distinct from regular Gallo-Frankish ones.
But we're talking small scale, and not migrating people moves there.
I did make one mistake -- they were defeated by a combination of Franks under king Childeric and Romans under Count Paul, not the Visigoths, which pinpoints when their settlement came to an end. I'm not sure how early it began, but this is an alternate history site, so if it wasn't in the early fifth century in real life, it can be on this thread.
Actually, there's some clues they had a small presence in Aquitaine, or at least noticable economical/cultural influence, so the mistake is quite meaningless here.
For the date of settlements, due to their nature, it's hard to distinguish regular raids made from Germania/Brittania directly, or trough outposts. I would think that, keeping the Viking parallel, they could have likely done such since the V century, maybe (but discontinued) since the IV.
Could large enough numbers of Saxons pile into Gaul for long enough to relieve the pressure on Celtic Britain?
Saxons were able to have a presence in Gaul, precisely because they settled Britain : Procope mentions, with the migration of Brittons in Armorica, moves from Angli and Frisii in Gaul from Britain. It's far for being the only mentions, the Early MA Saints lives in Normandy, while heavily hagiographied, mentionsr egularly raids and invasions of Saxons from there.
Without Saxon/Angle/Frisian, etc presence in Britain, the raids and settlement of these groups in Gaul may have likely be less important and short-lived.
I think anything's plausible in the fifth century; Visigoths, Burgundians, Asding Vandals, Siling Vandals, Alans, Suebi, Salian Franks and Ripuarian Franks had already migrated there, some moving on into Spain.
It's a common cliché on EMA AH actually : Migration Period doesn't mean that anything can happen for no reason whatsoever. The migrations of Western Germanic peoples especially usually are made along earlier raids and presence, depending on how they managed to organise.
Admitting the leagues and confederations are the same than OTL in the V century, the situation is kinda stuck : Franks have a lasting presence in Gaul not only at its borders but hinterland as well since the III century and represents huge numbers, while Saxons/Frisians, etc seems to have been numerically
Saxons, depsite having raided the shores, didn't settled the countryside like Franks did (more or less forced by Romans) and a most probably discontinued presence up to the V doesn't help.
On the contrary, they OTL had important settlement in Britain hinterland, maybe since the IV century.
You could butterfly that away, but I think it would have needed preventing Saxon rise in first place, in order to not let them the possibility to move in first place. But let's admit they settle more Gallic hinterland than OTL and not in Britain, or far less.
Being in a region more inhabited (it's hard to have clear numbers, but 1,5/2 against 5/6 millions seems to be a valid guesstimate) and relativly more cohesive is going to prevent the advantages Saxons had in Britain to maintain a more cohesive and dynamic settlement. They would have real competitors, not that willing to share, without talking about a more important "roman" resistance.
Not to mention Britons who moved into Brittany, which could be opened up for the Saxons if the Britons had no reason to go there.
Admittedly, you could have something like short-lived Norse presence in Nantes (that Saxons settled, or at least the countryside, OTL). But they may have been more bullied than OTL Bretons : Saxons OTL managed to keep a presence in Normandy/Loire as they were used as a defense against Bretons raids.
Maritime based Saxons (I don't see why they would have turned entierly terrestrial, at least at the beggining, when it was their main advantage) could have encountered serious Frankish threat and without support from close back land (as Britain was used OTL) would have a relative hard time.
Still, yes, it's possible to have S-E Brittany and Loire's mouth having a Saxon presence, being eventually known as "Armorica Saxonia" or something close to it.
I would think, however, that regarding the huge difference between Britain and Gaul in matter of population, military forces, etc that they would be more or less absorbated into Gallo-Frankish population around the VII century with all non-Frankish germanic populations.
