The only thing I found on Google was this WaniKani thread. Apparently the Chinese option can get you most of the way? Mac is the only platform I couldn’t find anything for, but mazec is available for both iOS/iPadOS and Android. Allegedly Gboard’s handwriting option may also work with Japanese on the Android version. It seems like right now only way to run a proper Japanese IME with handwriting recognition on a mac is to do it inside an emulator for any other platform.