Hey Gard,
Thanks for the helpful links, I fully understand your concern.
I can do away with the numpy and opencv installs through the (much more outdated) python-numpy and python-opencv debian packages respectively. However, dlib does not seem to have such a package yet and having to maintain that would be out of scope for me. I'm only a dlib user, not a contributor, and i don't think it would be my place to package it.
The absolute minimum install would only install pre-packaged dlib through pip or compile it from source.
However, dlib is available through pip and running that command would be idempotent. If apt install is aborted then pip will simply retry installing dlib the next time apt installation is attempted. If the pip dlib install goes through and postinst is called again pip will simply state that dlib is already installed.
(I wrongly hit "Reply" instead of "Reply All" in my last email, thanks for letting me know)
With kind regards,
Lem Severein