Probably about as clean as it gets
Hopefully the Hiragana mono kit will not be too expensive as sadly so many Hiragana kits have been lately.
The novelties are really interesting, first time I see anything like that.
-I don't know if the Spacebars kit is really necessary as there are no accent bars. And the base kit already has a 7u. Don't know if 6U are that popular and maybe people already have some white spacebars.
Maybe consider adding Alice support in base and removing the spacebars kit?
-Did you consider Japanese text on the mods, like GMK Masterpiece (but with hiragana/kanji like Kuro/Shiro). Or like what you are doing on the novelties with the text on the "side" of the keys.
Feels it could be interesting.