I have been using LanguageTool[1] for years as "an open source alternative to [old school] Grammarly". It doesn't do that fancy "make this text more professional" AI stuff like this or Grammarly can now do, but they offer a self-hosted version so you don't need to send everything you write to OpenAI. If all you want is a better spelling/grammar checker, I highly recommend it.
[1] - https://github.com/languagetool-org/languagetool