I agree that the factories are considered standard early play for japan no matter what turn you attack, and they are the reason that the US can consistently defeat japan. I used to buy one a turn almost every game, and have realized over time that the money draw is what is giving the USA a major advantage on the water it can use to eject you from SZ 6, so I changed my play and dont go for the factories until later, if at all.
Defending sz 6 can be done with carrier purchases and using planes you already have. Keep 1 carrier, 1 destroyer and maybe a cruiser on J1, add a carrier J2 and J3, keep bringing planes home for defense, which isn’t a big issue with the mainland as you sub in land units for air. This allows you to move enough South to keep your transports afloat and to more quickly bring in ground troops on the mainland.
Usually sz 6 looks like this at the end of their turn:
J1 1 carrier, 1 destroyer, 3-4 planes (counting Japan itself) - collect about 40 ipcs
J2 2 carriers, 1 destroyer, planes vary depending on U.S. (if they bought for Atlantic, no worries), collect 50+ ipcs
J3 4 carriers (if U.S. is active in Pacific), 1 destroyer, 8-11 planes
Usually J4 and later is when I’ll start adding subs and bombers because if its gone according to plan, J2 has Money Islands locked down, J3 is Malaya… with the benefit being that, by now, I’ve added 6 mechs and 2 inf/1art to the mainland army (mechs up north, slow movers in FiC) and income is around 60 ipcs at end of J3. If things are going well, on J4 you can invade the Soviet Far East and can easily spare the ground troops, to help gobble up some income if Germany can use the help, or push far enough into China to bomb Moscow, or just go hard at India but without having to commit your entire fleet.