Nested hugo module

I have a repo with some not-Hugo-related go modules (i.e., go.mod/go.sum in the root) and a Hugo site which is not in the root (it’s located at assets/site/). I want to take my custom Hugo theme out of that repo and use it as a Hugo module (just like what you’re doing in hugoDocs / gohugoioTheme).

I managed to have it working creating a nested go.mod/go.sum in the Hugo directory (assets/site/). I wasn’t able to make it work including the theme in the already existing root go.mod/go.sum. Can you tell me what’s the right approach here? Is it possible to have a single go.mod/go.sum in my main directory or the nested go.mod/go.sum is the way to go?