There are around some specialized (and commercial) fonts that are made explicitly top be used as 'engrave fonts' or 'outline fonts', but on the OpenSource world there are not much choices, except maybe for font cited above. Some fonts are decent even on the UTF-8 extension part (going further the ASCII set of letters and numbers), some are not so good. I have done in the past some experiment to use fonts, to make as example projection on two cylinders to make 3d fonts, see maybe: I think that some Wiki Page that gave some advice on what font is good and what is not very good to use as Shapestring or Engrave font will be interesting to have. More generally speaking OTF and TTF could be used in a cross platform way, other fonts, not too much. Some fonts are better than others to be used as example in Shapestrings, but there is no way to know what are good and what are not good, and what could be improved, Yes it is possible to modify glyphs and there are around some guides on how to do this sort of things. (they have some 'old fashioned' appearance)

Outline fonts - they are not a special category, but some of them are made with some care to not cross lines and could be used as example to define some 'cut out' on shapes and usually are also good to be transformed in shapestrings, like some 'military looking fonts' that resemble those used as stencils to mark military and shipping boxes.

Single stroke fonts - These fonts, are usually developed explicitely to be engraved (as example CamBam sticky fonts) discusse somewhere in this forum, they have some care (but sadly not 100%) to avoid to 'pass' over other glyphs and usually are composed by a single line, to be followed by the engrave tool.

Generic fonts - what is good to be visualized and printed on paper, usually they don't care to much about glyphs compositions so crossing and double strokes will appear (see further).

We could try to distinguish between three categories of fonts: I will try to generalize on some extent things, but for sure I will not be exhaustive and probably not very correct, but it is a start, feel free to contribute and signal wrong assumptions or conclusion. My intention is to collect some hints from user about fonts, and their use in FreeCAD.

To avoid as much possible to be OT, I've opened this thread.

Single line fonts for cnc