@timtsuga and I discussed this before as well.
https://community.glowforge.com/t/is-pdf-layer-extraction-without-acrobat-possible/16307
Tl;dr: You can extract topo information from usgs maps using inkscape, you simply need to dig down through the grouped objects to get to it. AD seems slicker but I’ve still not tried it.
One thing I like about your process is the ability to set custom contour intervals. That’s slick.