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