Basically I use Hotkey to trigger “palette” to another “palette” to achieve the Launcher mode effect. With this method you create a master palette to host 1st character “a-z” then another palettes to host the 2nd characters. If you use all the alphabets, you will need 27 palettes (1 master + 26 for all the alphabets)
The example below shows when tap “F” key alone and “F” + “D”
This macro live inside “Launcher mode master palette” which house all the 1st tap characters
This macro live inside “Launcher mode F+” which house all the characters after 1st “F” tap
Here is the macro link
Download from Dropbox
You will need to create 2 palettes yourself, one “Launcher Mode” which trigger by the “Hotkey” and 2nd one name “Launcher Mode - F+”
I am not sure if KM export those details when I export the macro.
With this method you can actually keep nesting the logic and have more then 2 click after trigger instead the limitation of 2 characters from Karabiner. Personally I use 2 hotkeys to trigger this workflow. One for all system command and one for per-apps and find 2 characters is enough for the job. I rarely use my modifier keys, expect shift for capitalize characters.
As this moment I am still using Karabiner. This post is just to explore the option when the day I end up have to move away from it. My wish list for KM will be invisible palettes 
cheers