You added contrib non-free in the last two lines, but this way you get the same repo twice and no contrib or non-free updates. You ought to have just added " contrib non-free" to the end of all pre-existing uncommented lines.
Anyway, I added this and a whole long catalogue of network and video drivers to the image yesterday, including iwlwifi, and uploaded the new version here. The new kernel modules have only been added to the 4.19.0-12 kernel (whose initrd now would terrify Godzilla by its sheer size). The 4.19.0-11 kernel is much leaner and can serve as a fallback if needed.
This thing still needs testing on different hardware to flesh out the inevitable errors and omissions, so please keep playing with it.