I finally found the NASM assembler.

https://nasm.us/index.php

I had heard that name before, many times, but somehow never looked into it. Weird. 🤨🤔

This is the kind of program I was looking for.

1. It is free software. Especially in the DOS ecosystem, free/libre software is a very scarce resource.
   
2. It’s a small command line program, not a huge behemoth.
   
3. Documentation appears to be well written.
   
4. It can even cross-compile DOS binaries from Linux.