Personally I find Banzai to be one of Japan’s best advantages as it is. It allows them to forgo expensive art and tank buys and just use Inf for ground wars.
My group uses full NAs too BAGM; for Japan we did the following based on LHTR:
Combined the Suicide NAs into one,
Altered the Fanatical Defenders to "Japanese Inf defend on a 3 in the first cycle of combat when defending a yellow territory. Japanese Inf on islands are not subject to Battleship bombardment.
and are still playing out various “new” NAs. Our current option up for testing is “Total war!”, allowing Japan to build an unlimited number of units on its home territory.
I also come up with something of a new Fanatical Resistance (Dug-In Defenders):
The Japanese introduced the tactic of endurance engagements intended to inflict maximum casualties. This tacic called Fukkaku included bunkers and pillboxes connected by tunnels.
All your infantry on islands and Japan are immune to shore bombardment and defend during the opening fire of combat.