ChatGPT is so useful for doing x86 assembly and low level osdev stuff. OSdev is general is very poorly documented unless you're willing to read absolute tomes worth of books. I wonder what it would look like if you assembled all the Intel reference books you'd need to program a modern x86 computer from scratch, including USB and Intel HD Audio and all that crap.
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:
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