This is a mighty undertaking, maps from way back then seem so hard to replicate these days.

I have no experience with Inkscape but looking at Germanus's map, seems like the mountains have more of a thin black outline instead of Edges Matte you applied, maybe try that? As for the inner texture, you might need to edit a replica of that lumpy surface if you can't find or replicate one. It appears that he painted the under layer with a brighter brown then went back and forth with a darker one, in a almost checkered pattern but following the "flow" of the mountains. I feel like I could get something similar in Photoshop or with alcohol markers on paper, but again, no ideia as it pertains to Inkscape.

Sorry if nothing I said helps, but I wanted to ask you if you plan on drawing the winds blowing from the heads. They add so much charm to the art. Good luck, and can't wait to see the finished product.