Using Fedora 35 here, still using i3. The trick is to modify /usr/lib/systemd/user/plasma-kwin_x11.service, here's mine.

    [Unit]
    Description=KDE Window Manager
    #Wants=plasma-kcminit.service
    PartOf=graphical-session.target

    [Service]
    ExecStart=/usr/bin/i3
    BusName=org.kde.KWin
    Slice=app.slice
    Restart=on-failure

The only downside is that you will need to wait a bit more for it to start.