Some of these CMakeLists.txt files assume that dependencies like gRPC and OpenSSL and whatever are available on the machine and can be found. This seems like a reasonable assumption when everything is under Nix, but could we include calls to FetchContent (https://cmake.org/cmake/help/latest/module/FetchContent.html) to fetch the dependencies in case they aren't on the machine to keep things more portable?

Yea, that doesn't seem too unreasonable. But there is a long term maintenance burden with doing this, as there's a need to maintain two configuration which may be out-of-sync. However, falling back to pulling from a repo doesn't seem too bad. I can create a task to do so.