Nuclide is a cool SDK for developing stand-alone games based on the Quake1 based FTE from the same developer that made FreeCS and related projects.
IMHO the FTE engine is way underrated (mainly because the developer can't be bothered to update the website or documentation 😓 ) and this should make developing with it much easier.
Edit: Actually to my big surprise the FTE website was recently updated 🤩