Several users such as @plantron have mentioned using a writing IME to be able to SRS writing kanji as well, but where can you find something like this for any platform? (e.g. iOS, iPadOS, macOS, Android, Windows, etc.)
Thanks!
EDIT: This discussion is primarily about iOS and iPadOS, but information about other platforms is welcome.
Here’s a summary:
Android - Free! Use the native keyboard or Gboard
iOS/iPadOS - $8.99 for Mazec or use the native Traditional Chinese keyboard (but no kana)
macOS - Some apps available (research this); native Traditional Chinese keyboard
Windows - Some apps available
For iOS and iPadOS, there’s no native software Japanese handwriting keyboard. A close alternative for kanji is the Simplified Chinese Handwriting keyboard; it can recognize Japanese character forms in addition to Simplified and Traditional Chinese. Interestingly, it can even recognize Japanese-only characters such as 働 and 峠.
If you want to write full sentences in Japanese including the kana, you can download a 3rd party keyboard like Mazec which is a paid app.
Back in my Galaxy S3 days, native support for Japanese from Samsung was also non-existent. For example, I could not change my phone language to Japanese, and adding a Japanese keyboard was a pain. By the time I got my Galaxy Note 4, they had fixed this. To be honest, I don’t know if it was a Samsung problem or an Android problem.