I've been thinking about this, I did license one of my library [1] as LGPLv3 before. I really don't know what to do for now [2].
I'm not a fan of making libraries in a single header, I know it makes it easy to just copy, paste, and use. I designed libpgn to have small seperate 'modules' each serving functions (pgn_metadata_*, pgn_moves_*, etc) to parse a small portion of the file, that way the root pgn.h just calls and builds the block with all the small parsers.
[1]: https://en.wikipedia.org/wiki/Static_site_generator (ex: Hugo, Jekyll)