As an alternative there is LanguageTool[†], for which you can run your own server so the data isn't given to an extra party at all. We've been using it in [DayJob] for a while, and I use it at home too, and it does a decent job.
From my relatively experience of Grammarly (I had an account a couple of years ago) a self-hosted instance of LT is slightly better than Grammarly "free" but doesn't have the extra analysis offered by Grammarly's paid accounts.
I do this too (running it in a docker container on my server to mutualise usage across several devices) and it suffice for catching the main errors and typos. Quite happy with it.
Can indeed be recommended as substitute to grammarly free.
From my relatively experience of Grammarly (I had an account a couple of years ago) a self-hosted instance of LT is slightly better than Grammarly "free" but doesn't have the extra analysis offered by Grammarly's paid accounts.
[†] https://languagetool.org/