[Edit: Thank you all so much for the replies! I learned a lot researching your suggestions. Am now leaning toward Double-X or one of the suggested color stocks.]

The artistic choices for the project have been determined. My DP and I would love a 16mm film stock that responds and develops like Ilford HP5 Plus, for all its strengths and weaknesses.

I like its contrast, softness, and grain as-is, when not pushed or pulled. I like its handling of highlights and shadows significantly better than, say, Kodak Tri-X 400, which I find to plunge far too deep too quickly into the shadows, to be too contrasty in general, and a tad too sharp. HP5 Plus seems to have better dynamic range, too.

We still want a very dark film, so would likely not want something less contrasty, either.

I've shot lots of stills on film (but am new to development), I'm intermediate or better at digital cinematography (newish to color grading), but consider me a total beginner in the world of 16mm motion picture.

Looking forward to your suggestions. Thanks!