Normally I would go for transports.
That would be buy two on J1 (fleet total is 3 or 4 now) and on J2 buy one or two more. Coordinate this with ground unit builds in Japan to fill all the transports and do the infantry push to Moscow.
In the current game, the Allied/UK player (RJ), placed his bomber within reach of Japanese home waters (Novo) and moved his Australian submarine up within reach also (Coral Sea). This put any unprotected transport build in home waters at risk. The added cost of either diverting fleet assets to cover them or building cover and still over building to deal with losses actually made me move toward factories on the mainland. Add in that he moved most of his India forces to Egypt and skipped on the UK1 DD vs Trans option.
I built two, one in FIC and one in Manchuria. Sure, they will never out build the home islands but now my infantry push into Russia is independent of any transport issues. For 30 IPC I have the equivalent of three transports that won’t sink. I would have spent 24 for those same three transports plus a DD to protect them (12 more) then probably another transport to replace loses (8 more). So it was two factories at 30 IPC or 4 Transports and a DD at 36 IPC. The factories are at least one turn closer to the front so discount a turn of Infantry in the pipeline (6X3=18IPC) and now I have 30 IPC for factories vs 54 IPC for transports, DD, and infantry to fill the pipeline. I will have to build AA guns for the factories to keep the SBR from becoming annoying but they will also be able to move up with the front lines making my kill stacks harder to hit.
As an additional bonus, I can still add transports to the fleet in later turns and if they are not moving gear to the mainland they are now available to support fleet operations / threaten Alaska/Austrialia/Africa. This decision meant I could send the homeland transport to Hawaii with the fleet for a low odds invasion of Hawaii and the Kwangtung transport is busy supplementing the mainland, ferrying Inf from the islands, and joining the fleet facing India.
I won’t claim these are perfect moves but faced with an opponent who was looking to disrupt the J1 transport build, it was not a bad counter. At least, so far…