@Ben, I’m not sure whether it is a good idea to pick separate projects instead of sync’ing all of them together. I can imagine there is a lot of dependencies, so you will probably end up needing more then 100 or 200 projects. Syncing all of them by hand sounds pretty insane
If you want to save time, download only shallow copies and limit it only to the needed branch:
repo init --depth 1 -u <URL> -b <BRANCH> -m <MANIFEST>.xml
repo sync -c