Exactly, it's pretty quick to swipe to the wanted kanji/word.
But katana is a bit more annoying (Latin already work but with Caps lock only)
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Another suggestion would be a Japanese style input layout.
Using a kind of T9 keyboard layout would be perfect I think.
The thing is that you would have the keys like this :
[1 あ ] [2 か ] [3 さ ]
[4 た ] [5 な ] [6 は ]
[7 ま ] [8 や ] [9 ら ]
[‘' ° ] [ わ ] [、 。]
This is most common (basic cell phones) input method in japan,
then you have the choice or you hit quickly multiple times to switch between あいえう on the first key
or you swipe (if you have smartphones, that's how OpenWnn&Others work.)
As I said a Japanese Layout is the second stuff that would be really nice to see after the katakana/hiragana switch support.
Hope this will help you somehow lol.
Good luck with everything.
Other suggestion : I saw that you added a lot of special character from Japanese keyboard

(that's really nice)
I'm just missing one is the _ underscore, who is close to twice the size of the latin one.
if you can add it to the 3 little dots key('d') it would be awesome.
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A little bug (not important imho)
compact and compact/t9 don't work for Japanese input, maybe you should just disabled them for that language.
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More little bugs :
I've the Smiley key always shown, since I edited the smiles they are a bit longer than the default ones and the problem is that they overlap on the other keys on the keyboard. (it's like the text of the smile is centered on the "smile key" and if you add a smile that is too large then it goes outside of the screen and on the keys on the left of the "smile key"
(don't know if I explained myself clearly, otherwise I can take a screenshot).
More infos : Only seen on portrait mode, in landscape the smiley key is larger so no overlay.
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More bugs :
On landscape mode the key (123) is pressed (down) all the time.
it doesn't matter it's only a visual bug.
Thank you for everything,
The bugs I mentioned here are really not important, don't bother yourself too much with it if you have more important stuff to do.
The only real little thing that would retain a Japanese from using this keyboard would be the katakana feature missing and maybe the layout, IMHO.
Best regards,
Yvan.