I loved this action. Thank you!
(I was using codecov and was getting angry because it always gave errors when sending the result to their server.)

When refactoring the workflow I would follow the same path that nedbat used here which is to store each htmlcov in a repository (I would store it in a branch like you did).
Obviously using your action is much better even without the history, because you have the comment and that is very good and simplifies the process a lot.
In this action, you save the most recent htmlcov in a branch, so I hope it won't be complicated to expand and save the history.