Almost all of it is GUI now. The only terminal step is installing the packages (one line):
sudo dnf install fcitx5-mozc kcm-fcitx5 fcitx5-autostart
Then log out and back in, and the rest is clicking:
1. System Settings > Keyboard > Virtual Keyboard > pick "Fcitx 5" (on Wayland) > Apply.
2. Right-click the keyboard icon in the tray > Configure (Input Method Settings) > Add Input Method > search "Mozc" > Add. KDE's own guide suggests adding "Keyboard - Japanese" or keeping your English layout above it.
3. Ctrl+Space switches between English and Mozc in any app.
If an app ignores it, fcitx5-diagnose shows what's missing. Sources: Fedora wiki page I18N/Fcitx5 and linux.kde.org/docs/cjkv-input. You're right that an installer prompt for this would be nice.
