Ok, the point about the luxury trade is the first clear and logical incentive I've seen.
I get how that might incentivise Japanese merchant-adventurers to pursue bluewater navigation in the China seas, and, like the Portuguese and Dutch carve out and defend trading enclaves. This is probably more feasible than centralized state action.
But I'm still not sure how that gets them to the Indian Ocean. Were Indian luxuries in demand in Japan? I was under the impression that the vast majority of the luxury trade was with China.
The thing is, once you get in the country trade, it is useful to trade all over. It was a vast network with China wanting silver and spices, Indonesia wanting other spices and stuff from China, etc... The Indonesia+China sea could have been considered a world economy apart but it's good to diversify your investments, with silk and cotton for example.
Japan is a massive market by itself, with 26M people registered in 1721. Since it was still a premodern economy, we can imagine it stayed relatively stable. That means a lot of interior demand for spices and all. After a while, the copper and silver from the mines might not suffice, so you need the country trade, so you need to travel, etc...
Also an advantage that the Dutch certainly had was their leveraging of corporate structures to finance their expeditions. This enabled them to outstrip by far Portuguese efforts. I suspect that continued successful trading colonialism needs this sort of commercial sophistication.
Also completely funneling the Portuguese trade. It's been said that the Dutch corporation was simply an incremental improvement on the Portuguese structure, not something entirely different