Aside from kanji, Japanese has two phonetic alphabets. Their language is complex, but it also seems to have a lot of redundancy built in, even if romaji doesn't count as an unofficial extra Japanese alphabet by now.

Quora has an illustration[0] of a keyboard with english and hiragana characters mapped, and conversion to kanji by software. It seems awkward, but not to the degree that it would stifle adoption considerably. Admittedly, this is a naive view on my part.

[0]https://www.quora.com/How-do-computer-users-type-in-Japanese