This is really a problem because even in the Archicad Channel video (as you showed above) there are lines of varying thickness as you may have noticed.
I have some workaround for myself, but I don't know if it will suit you because it's a bit complicated.
But looks like the picture below.
Workaround is based on the following:
I turn off Skin End Line only for core wall material.
Then I add 0.1mm thick air space on the inside of the composite wall and wrap it to the outer edge of the window in Window Settings Dialog>>Wall Closure>>Inside Face, creating a Skin End Line where I need it.
I don't know if such a workaround will be useful for you, but sometimes it helps me for some cases.