Unfortunately, it's not possible for Zotero or any other tool to reliably automate conversion to sentence case, since a computer can't know for sure that something is a proper noun. (Think, for example, of the word “united”, which can be an adjective, verb, name of a company, or part of a country name.)
That's why this has to be a manual step that you then correct.
See Sentence Casing: That's why this has to be a manual step that you then correct.