I recently tested Swiftkey after Typewise is sadly abandoned. It's sooooo much better than the stock keyboard. Not only is the auto-correct working incredibly well (garbage like witjoit is correctly transformed to without, which Apple Keyboard can't), Swiftkey also manages multi-language typing astonishingly well. Last but not least, I can customize it. I am also not signed in to my account, so no settings or whatever is stored on Microsoft servers.