During Brittany's resistance to Frankish rule they consistently defeated the Frankish invaders
It's a bit more complex : Brittany, as Saxony, was part of the "outer" circle of Frankish influence. We know, for instance, that Brittons mercenaries and auxiliaries were particularily used in Neustria (at the point Armorican Saxons were ordered to Brittanize themselves).
As for consistently defeating Frankish advance...I think you might exagerate a bit. It's true that Franks never really managed to go for Lower Brittany (altough they did intervened, and not only militarily, such as Judicaël refusing a royal title and acknowledging Dagobert's suzerainty), they did managed to secure the eastern part for themselves (the
March of Brittany).
Generally speaking, when Francia was reasonably strong, Brittany was part of the Frankish hegemony. It of course was challenged in the VIIth before returning to status quo with the Carolingian rise.
It changed a bit in the late IXth century, mostly in the wake of the decline of Carolingia, with Brittons being unified (before hand, you basically have three polities), Carolingians fighting each other, and Norse raids really making a number on their authority in North-West wFrance. The Treaty of Compiègne basically gave Brettons 1/3 of modern Normandy and part of Anjou and Maine (basically pulling the same move that it was made later with Normans). Interestingly this expension made Brittany more tied and dependent from the Carolingian world it ever was, and is one of the causes (with Vikings raids and takeover) of the decline of independent Brittany.
due to alleged superior cavalry tactics.
Breton cavalry tactics were more or less basic : it wasn't a shock cavalry, but rather an harassing band made of mounted javeleeners that searched to break ranks, supporting a semi-guerilla tactics (Bretons infantrymen painting their shields blacks for night warfare, for instance). It arguably changed in the late IXth century, when Breton army frenchified (see above).
Had Bretagne the numbers or the will they may have expanded further?
Neither.
Number disprency is hard to overestimate, and only grew with time. (We're talking of, with a minimal guesstimation, 6 millions inhabitants in Merovingian Gaul, maybe up to 8 in the latter periods); One can argue that Britto-Romans presence in Armorica could have been significantly greater with a Wisigothic defeat at Déols, but I think it would have essentially reinforced the Britto-Gallo-Frankish continuum you can observe at this moment : at best, Bretons would have underwent an earlier Gallo-Romanisation.
As for the will : Bretons were part of the Frankish sphere, and while enjoying a great autonomy at worst, were dependent on the situation in Francia, with the kicker that more powerful and expensionist they could be, more tied up to Francia they were to secure their territories or even authority (it's why Brittany eventually re-entered a clear French suzerainty in the Xth)
