AFAIK the issue with using submodules is you still need the rights to pull the other source repo. However, you can use submodules or LFS to pull a specific build artifact from a build artifact repo or source instead of the source repo, which provides a neat way to manage the dep without fattening the main repo and allows the source repo to be kept separate and high security. I'd certainly do this before changing RCS/VCS solutions. That said, reverse engineering has become relatively trivial in the AI age so the practical utility of providing built rather than source elements is dropping.

You'd have the non-NDA, core stuff in the submodule. The submodule can then be referenced by the big, driving NDA repo. And maybe another repo with a non-NDA, likely simpler "shell" that lets non-NDA devs work with the codebase.

If you need to NDA the core stuff instead and thus can't pull it as a submodule, the only thing I can think of is to pull the core as binary/compiled artifacts.

You can set up submodules to not pull the other source repo by default tho