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.)
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