I make trail maps that can be useful to hikers and mountaineers. Some of the areas I map have outstanding mountain scenery, but are located in areas where local regulations "frown" on independent, non-official mapmaking. To the detriment of the outdoor community, unfortunately.
My current goal is to add contour intervals to my maps. This would greatly increase their useability. So far, I have been using OpenTopoMap [dot] org as a source. OpenTopoMap solves one problem, namely, the ability to grab contour images for areas of interest to me. Or as I might put it, for any place on earth, so as to leave the specific locale unnamed.
But my methods are primitive. Basically, import the image to a layer in LibreOffice Draw and manually add trails, peaks, features, etc. based on my local knowledge of an area. With my current approach, I lack suitable control over the contours. Many times they are too close, rendering the map unreadable. I need the ability to choose the contour interval, select the positioning of altitude lettering, choose colors, etc.
I suspect I may need to learn how to use GPS software that can import contour data from a suitable source, but my current level of knowledge is sparse. Basically I need some direction on how to start. I would appreciate suggestions on software, sources of data, for someone at a beginning level, but who aspires ultimately to make high-quality maps.
Are there alternative sources of contour data that would be better suited to my purposes than OpenTopoMap [dot] org? Bear in mind that my area of interest is "anywhere on earth". Helpful suggestions welcome, and thanks.