Ugh, I didn't know that SHA-1 submodules wouldn't be supported in SHA-256 repos. That changes the transition from painless to a major dumpster fire. Having to maintain converted forks, and use different hashes from upstream is going to be a mess.
I used to be one of the more vocal defenders of C++ but honestly, if the reason you’re defending git submodules is because there’s no better option in C++; then you’ve basically already lost the argument.
Nearly every other language has found a better alternative. Even Go has, and that’s largely mocked for its prehistoric approach to modern programming language design.