I take it that the LFF font format is monospaced and that TTF2LFF only created outlines. I created large sets of AutoCAD fonts in the past and worked with an application that created filled fonts using lines and arcs from Truetype files. I'm wondering is there's any interest in that for LibreCAD users.
I think it is not necessarily monospaced, compare font iso and iso3098:
I don't know if it is possible to use filled fonts with lff, I am not a developer. There have been discussions about implementing use of ttf fonts, but the developing effort was considered as too much.