They wont defeat Big Mom.
BM will retreat after Kaido is defeated, and shell make a short aliance with black beard in elbaf, immo Big Mom will be a gatekeeper in elbaf vs Luffy and the others, there is were she will be defeated.
Why would BM make an aliance with bb, because she does not want Luffy to become pk, after Luffy defeats Kaido, BM will know that Luffy will do it, so the last solution of her is to ally with someone as strong as Kaido or stronger again.
Elbaf will be much shorter than Wano, Bm will fall in like 10 chapters or so. And than the real arc vs Black Beard will start, the hunt that will reunite all the poneglyphs.:cheers: