You don't even need to make a "level", just make cool rooms or whatever. Don't bother with mobs or items, just geometry and lighting. I'll create any needed textures so just use placeholders, or you can if you can handle creating matching grayscale albedo+normal map textures yourself.
Seriously this is basically the last call to try to make something. Trenchbroom is easy and FOSS. All the tools are FOSS. Here's a tutorial: https://www.youtube.com/watch?v=gONePWocbqA&list=PLgDKRPte5Y0AZ_K_PZbWbgBAEt5xf74aE&index=1
Use any WAD pack, in reality the wad textures are not used and just serve as material names which include stuff like normal maps. Here's a good prototype WAD: https://github.com/jonathanlinat/quake-leveldesign-starterkit/raw/master/wads/prototype_1_3.wad
