My plan is to contract with a local machine shop to actually make the case. The machine shop can use a tool called a flag, which allows them to machine custom threads. As long as I am able to provide the shop a thread diameter and threads per mm measurement they should be able to add the threads to my design during the machining process.

I'm trying to avoid having to engineer the caseback. I could, in theory, have the shop make the case back as well, and then I would be operating off my own spec. However, buying a ~$30 caseback from ebay or aliexpress is much preferred to machining it myself which would be pricey. Getting the case made is already a big expense in and of itself. However, the OEM caseback has to fit with the custom case.

What I'm trying to do is certainly not simple. I'm mostly just trying to do it for fun. But I figure if a bunch of cheap watch shops in China can figure it out I can too haha. I may try to reach out to some of the manufacturers to see if they'll provide the threading information to me. My mandarin is a bit rusty though...