When doing the split the --rejoin
option can be used, which seems to exist exactly to reuse the splits that have already happened.
--rejoin::
This option is only valid for the split command.
After splitting, merge the newly created synthetic history back into your main project. That way, future splits can search only the part of history that has been added since the most recent --rejoin.
From https://github.com/apenwarr/git-subtree/blob/master/git-subtree.txt