Why not modifiers with text, japanese text?
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https://geekhack.org/index.php?topic=97699.0
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In term of aesthetics, it looks better for japanese only alphas to match the icon only modifiers. Each key has a single legend. It looks simple this way. It will be too busy for the alphas to have both text, japanese legends, however the modifiers keys only have icons.
Since there are several english/japanese alphas keysets, e.g., laser, red samurai, serika, ebpt black japanese, and many more are coming, e.g. striker, etc, but just a few ones with only japanese, e.g. laser kobe, samurai pure Hiragana. I think it makes sense for OP to make this set alpha with japanese only so that it's more special.
As a side effect of only japanese characters playing the role of legends, it becomes some kind of novelties legends for users who don't know/speak japanese. Some colemak/dvorak fans would be happy to buy it too.