I mean, if you wanna speed run it at MAX speed, you can just buy all leves items directly from MB, right?
You can also buy the raw mats from MB for making Firmament collectables, which basically gives insane EXP all the way to level 80 (might wanna do this before burning leves, which have 100 allowances only).
You can also buy GC provision mission items directly from MB too.
Who knows how much all these will cost you... perhaps instead of earning 15 million along the way, you end up spending 15 million gil? I have no idea!