So far as I can tell, there doesn't seem to have been a great electoral variation between Franconia and Bavaria proper. Is this normal? I'm aware that there's still a fair degree of cultural difference between the two regions and that Franconia has a much larger Protestant population, but I don't know if that factors through into voting intention at all.
Well if you squint you can still perceive a certain variation between Franconia and the rest of Bavaria, but fundamentally yes, both are an almost endless landscape of safe CSU districts and have been for quite a while [outside of the two big cities of Nuremberg and Munich of course].
Upper Franconia in particular, but also Middle Franconia around Nuremberg in good years, used to be fertile enough ground for the SPD to win direct mandates, but those times are long gone; Coburg and Hof still went red in 1998 but that's about it. Perhaps it lingered on more at the local level - the city of Bayreuth had only SPD mayors continuously from 1948 to 2006 for example.