Many thanks for the detailed description of your workflow. That confirms my new plan (in my previous post).
The old plan, in the earlier posts, will not work. Which means the documentation https://docs.moodle.org/37/en/Git_for_Administrators#Installing_and_maintaining_contributed_extensions_using_Git_submodules, also quoted in https://moodle.org/mod/forum/discuss.php?d=396476#p1598412, needs an overhaul. I won't touch it, don't claim to understand enough.