Can you not use a desktop software to correct your spellings, fix your grammar and reformulate sentences? I think even something like Microsoft Word is going to be a good bargain for this. If you want a free tooling Emacs with some plugins can be great. A quick search gave me https://simpleit.rocks/lisp/emacs/writing-in-emacs-checking-... Exploring this space may give more results for free tooling that runs your desktop and sends your data to noone.