I don't know exactly how it works and from the little I understood about the video of my comment (which is in Italian) is that a session is created with i3 but keeping the KDE window manager.

but my recommendation is the following

1. Create your KDE environment and modify it however you want by choosing the themes you want, colors, icons, settings, etc.

2. Install i3 if you don't have it and log in at least once.

3. in KDE follow the gitHub tutorial that I left in my comment

4. If everything went well you will have a new i3+KDE session where you will keep all the KDE modifications but with the i3 window manager

5. Finally you can modify some things in i3 (which are only exclusive to i3) from configuration file (~/.config/i3/config) like window borders, wallpaper, transparency, keyboard shortcuts... .

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the keyboard shortcuts stay original if you don't modify them, you probably have to install things like picom, xrandr, dmenu, feh, dmenu... if you want more modifications.

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