This is the best summary of the topic I've read. Having used all three (submodules, monorepo, manyrepo) the only thing I can say with any certainty is - don't use submodules. The mono/manyrepo decision is not as clear cut but your description nails it.

Edit: submodules IS a viable solution for truly third-party repos over which you have no control and don't expect to ever edit.