As others have said, Grammarly is inaccessible with the keyboard and with screenreaders.  For a sighted mouse user, it works great, it highlights grammatical errors in many places, including forms you are filling in on the web.  A similar service, ProWritingAid, has a screen reader accessible interface.  It's not free, but worth considering if you're after a full featured grammar checker: https://prowritingaid.com/en/Analysis/Editor?screenReaderFriendly=true

It is completely inaccessible with any screen reader. Your best option would be to change the grammar checker in MS Word to also check for styles, not just grammar. That's the only accessible avenue for grammar checking because all other third party solutions are not screen reader friendly.