Used items that you plan to take with you when you leave usually aren't a problem and their current market valuations can be 10% or less of their prices when new (for example, clothing).

For some countries, including Japan, the stuff that might worry me would be certain ingredients in medicine that's OTC here but possibly forbidden or considered to be a serious scheduled narcotic in some foreign countries. There was a case recently involving oxycontin in Japan, although some mid eastern countries and Australia are especially strict on this.

I agree that your gear might raise questions of needing some sort of a work visa.

Certain stuff might cause suspicion that you plan to sell it during your trip. Items of this sort would be new electronics and anything which is especially expensive in Japan, perhaps due to high duty rates or other protectionist policies. Multiple identical or similar items or things in the original packaging could be viewed as red flags.