Ibaraki leveling or focus on the Onigashima event?

Why not both?
You don't need embers to raid a bunch. This event is two weeks long - you'll have plenty of time to farm the raid fights for DP, and plenty of time to farm.

Focusing on Banoni isn't a bad idea if you plan to use her in the event because of the damage buff, but you can totally raid in your AP downtime, then focus on shop farming after the raids are cleared and Banoni is sufficient level.

You start netting some golden fruit in the DP ladder around 70mil, so if you spend the first few days on lower BP raids, you can focus on raising Ibaraki and getting to that first fruit, then you can REALLY raise Ibaraki once you start using her in those raids.

Get enough of the corals (mountain pass for best farming for now) to get one copy of the Sumo CE, and then spam the raid boss (do each stage to gauge how strong your team), and use the AP on ember farming. I don't know if this'll be any help, but my strategy for beating 3BP 6 million HP Rashomon was to bring a Single Target NP servant, AoE NP servant, Waver support, Hans, Mash, and some random servant for bond point farming. The idea was to blow your load the moment you had a NP brave chain of the single target DPS, stacked with the crit buffs from Waver and Hans (I used the combat uniform mystic code to sap out Waver or AoE servant with Hans equippd with the golden carp CE that gave crit stars). You can clear this event without doing the 6 million HP boss unlike Rashomon, so if you can't clear it, don't worry too much. If you can deal 1.5 million dmg on the 3 million HP boss, then that's fine too.